On 27 September 1913, Dönitz was commissioned as an Acting Sub-Lieutenant (Leutnant zur See). When World War I began, he served in the light cruiser SMS Breslau in the Mediterranean Sea. In August 1914, Breslau and the battlecruiser SMS Goeben were sold to the Ottoman navy; the ships were renamed the Midilli and the Yavuz Sultan Selim, respectively. They began operating out of Constantinople (now Istanbul), under Rear Admiral Wilhelm Souchon, engaging Russian forces in the Black Sea. On 22 March 1916, Dönitz was promoted to Navy First Lieutenant (Oberleutnant zur See). When Midilli put into dock for repairs, he was temporarily assigned as airfield commander at the Dardanelles. From there, he requested a transfer to the submarine forces, which became effective in October 1916. He served as watch officer on U-39, and from February 1918 onward as commander of UC-25. On 5 September 1918, he became commander of UB-68, operating in the Mediterranean. On 4 October, this boat was sunk by British forces and Dönitz was taken prisoner on the island of Malta.

The war ended in 1918, but Dönitz remained in a British camp near Sheffield as a prisoner of war until his release in July 1919. He returned to Germany in 1920.

During the interwar period, Dönitz continued his naval career in the naval arm of the Weimar Republic's Armed Forces (Reichswehr). On 10 January 1921, he became a Lieutenant (Kapitänleutnant) in the new German Navy (Vorläufige Reichsmarine). Dönitz commanded torpedo boats by 1928, becoming a Lieutenant - Commander (Korvettenkapitän) on 1 November of that same year.

On 1 September 1933, Dönitz became a full Commander (Fregattenkapitän) and, in 1934, was put in command of the cruiser Emden. Emden was the ship on which cadets and midshipmen took a year long world cruise in preparation for a future officer's commission.

On 1 September 1935, Dönitz was promoted to Captain (Kapitän zur See). He was placed in command of the 1st U-boat Flotilla Weddigen, which included U-7, U-8, and U-9.

During 1935, the Weimar Republic's Navy, the Reichsmarine, was replaced by the Navy of Nazi Germany, the Kriegsmarine.

Throughout 1935 and 1936, Dönitz had misgivings regarding submarines due to German overestimation of the capabilities of British ASDIC. In reality, ASDIC could detect only one submarine in ten during exercises. In the words of Alan Hotham, British Director of Naval Intelligence, ASDIC was a "huge bluff".

German doctrine at the time, based on the work of American Naval Captain Alfred Mahan and shared by all major navies, called for submarines to be integrated with surface fleets and employed against enemy warships. By November 1937, Dönitz became convinced that a major campaign against merchant shipping was practical and began pressing for the conversion of the German fleet almost entirely to U-boats. He advocated a strategy of attacking only merchant ships, targets relatively safe to attack. He pointed out that destroying Britain's fleet of oil tankers would starve the Royal Navy of supplies needed to run its ships, which would be just as effective as sinking them. He thought a German fleet of 300 of the newer Type VII U-boats could knock Britain out of the war.

Dönitz revived the World War I idea of grouping several submarines together into a "wolfpack" to overwhelm a merchant convoy's defensive escorts. Implementation of wolf packs had been difficult in World War I owing to the limitations of available radios. In the interwar years, Germany had developed ultra-high frequency transmitters which it was hoped would make their radio communication unjammable, while the Enigma cipher machine was believed to have made communications secure. Dönitz also adopted and claimed credit for Wilhelm Marschall's 1922 idea of attacking convoys using surface or very near surface night attacks. This tactic had the added advantage of making a submarine undetectable by sonar.

At the time, many — including Erich Raeder — felt such talk marked Dönitz as a weakling. Dönitz was alone among senior naval officers, including some former submariners, in believing in a new submarine war on trade. He and Raeder constantly argued over funding priorities within the Navy, while at the same time competing with Hitler's friends, such as Hermann Göring, who received greater attention at this time.

Since the surface strength of the Kriegsmarine was much less than that of the British Royal Navy, Raeder believed any war with Britain in the near future would doom it to uselessness, once remarking all the Germans could hope to do was die valiantly. Raeder based his hopes on war being delayed until the German Navy's extensive "Z Plan", which would have expanded Germany's surface fleet to where it could effectively contend with the Royal Navy, was implemented. The "Z Plan", however, was not scheduled to be completed until 1945. Dönitz, in contrast, had no such fatalism and set about intensely training his crews in the new tactics. The marked inferiority of the German surface fleet left submarine warfare as Germany's only naval option once war broke out.

On 28 January 1939, Dönitz was promoted to Commodore (Kommodore) and Commander of Submarines (Führer der Unterseeboote).
In September 1939, Germany invaded Poland, Britain and France declared war on Germany, and World War II began. The Kriegsmarine was caught unprepared for war, having anticipated that the war's outbreak would be in 1945, not 1939. The Z Plan was tailored for this assumption, calling for a balanced fleet with a greatly increased number of surface capital ships, including several aircraft carriers. At the time the war began, Dönitz's force included only 57 U-boats, many of them short range, and only 22 oceangoing Type VIIs. He made do with what he had, while being harassed by Raeder and with Hitler calling on him to dedicate boats to military actions against the British fleet directly. These operations had mixed success; the aircraft carrier HMS Courageous and battleship Royal Oak were sunk, and battleships HMS Nelson damaged and Barham sunk, at a cost of some U-boats, diminishing the small quantity available even further. Together with surface raiders, merchant shipping lines were also attacked by U-boats.

On 1 October 1939, Dönitz became a Rear Admiral (Konteradmiral) and "Commander of the Submarines" (Befehlshaber der Unterseeboote, BdU, the German equivalent of ComSubPac or ComSubLant); on 1 September the following year, he was made a Vice Admiral (Vizeadmiral).

By 1941, the delivery of new Type VIIs had improved to the point where operations were having a real effect on the British wartime economy. Although production of merchant ships shot up in response, improved torpedoes, better U-boats, and much better operational planning led to increasing numbers of "kills". On 11 December 1941, following Adolf Hitler's declaration of war on the United States, Dönitz immediately planned for implementation of Operation Drumbeat (Unternehmen Paukenschlag). This targeted shipping along the East Coast of the United States. Carried out the next month, with only nine U-boats (all the larger Type IX), it had dramatic and far reaching results. The U.S. Navy was entirely unprepared for antisubmarine warfare, despite having had two years of British experience to draw from, and committed every imaginable mistake. Shipping losses, which had appeared to be coming under control as the Royal Navy and Royal Canadian Navy gradually adapted to the new challenge, skyrocketed.

On at least two occasions, Allied success against U-boat operations led Dönitz to investigate possible reasons. Among those considered were espionage and Allied interception and decoding of German Navy communications (the naval version of the Enigma cipher machine). Both investigations into communications security came to the conclusion espionage was more likely, or else the Allied successes had been accidental. Nevertheless, Dönitz ordered his U-boat fleet to use an improved version of the Enigma machine (one with four or five rotors, which was even more secure), the M4, for communications within the fleet, on 1 February 1942. The German Navy (Kriegsmarine) was the only branch to use the improved version; the rest of the German armed forces (Wehrmacht) continued to use their then current three - rotor versions of the Enigma machine. The new system was termed "Triton" ("Shark" to the Allies). For a time, this change in encryption between submarines caused considerable difficulty for Allied codebreakers; it took ten months before Shark traffic could be read (Ultra codebreaking and Cryptanalysis of the Enigma).

By the end of 1942, the production of Type VII U-boats had increased to the point where Dönitz was finally able to conduct mass attacks by groups of submarines, a tactic he called "Rudel" (group or pack) and became known as "wolfpack" in English. Allied shipping losses shot up tremendously, and there was serious concern for a while about the state of British fuel supplies.

During 1943, the war in the Atlantic turned against the Germans, but Dönitz continued to push for increased U-boat construction and entertained the notion that further technological developments would tip the war once more in Germany's favour while briefing the Führer. At the end of the war, the German submarine fleet was by far the most advanced in the world, and late war examples, such as the Type XXI U-boat, served as models for Soviet and American construction after the war. These, the Schnorchel (snorkel) and Type XXI boats, appeared late in the war because of Dönitz's personal indifference, at times even hostility, to new technology he perceived as disruptive for the production process. His opposition to the larger Type IX was not unique; Admiral Thomas C. Hart, who commanded the United States Asiatic Fleet in the Philippines at the outbreak of the Pacific War, opposed fleet boats as "too luxurious".

Dönitz was deeply involved in the daily operations of his boats, often contacting them up to seventy times a day with questions such as their position, fuel supply, and other "minutiae". This incessant questioning hastened the compromise of his ciphers, by giving the Allies more messages to work with. Furthermore, replies from the boats enabled the Allies to use direction finding (HF/DF, called "Huff - Duff") to locate a U-boat using its radio, track it, and attack it (often with aircraft able to sink it with impunity).

Dönitz wore on his uniform both the special grade of the U-Boat War Badge with diamonds, and his U-Boat War badge from World War I, along with his World War I Iron Cross 1st Class with World War II clasp.

On 30 January 1943, Dönitz replaced Erich Raeder as Commander - in - Chief of the Navy (Oberbefehlshaber der Kriegsmarine) and Grand Admiral (Großadmiral) of the Naval High Command (Oberkommando der Marine). His deputy, Eberhard Godt, took over the operational command of the U-boat force. It was Dönitz who was able to convince Hitler not to scrap the remaining ships of the surface fleet. Despite hoping to continue to use them as a fleet in being, the Kriegsmarine continued losing what few capital ships it had. In September, the battleship Tirpitz was put out of action for months by a British midget submarine. In December, he ordered the battleship Scharnhorst (under Konteradmiral Erich Bey) to attack Soviet bound convoys, but she was sunk in the resulting encounter with superior British forces led by the battleship HMS Duke of York.

In the final days of the war, after Hitler had installed himself in the Führerbunker beneath the Reich Chancellery in Berlin, Hermann Göring was considered the obvious successor to Hitler, followed by Heinrich Himmler. Göring, however, had infuriated Hitler by radioing Hitler in Berlin asking for permission to assume leadership of the Reich. Himmler also tried to seize power himself by entering into negotiations with Count Folke Bernadotte. On 28 April, the BBC reported that Himmler had offered surrender to the western Allies and that the offer had been declined.

In his last will and testament, dated 29 April, Hitler surprisingly named Dönitz his successor as Staatsoberhaupt (Head of State), with the title of Reichspräsident (President) and Supreme Commander of the Armed Forces. The same document named Propaganda Minister Joseph Goebbels as Head of Government with the title of Reichskanzler (Chancellor). Further, Hitler expelled both Göring and Himmler from the party.

Rather than designate one person to succeed him as Führer, Hitler reverted to the old arrangement in the Weimar Constitution. Hitler believed the leaders of the German Army (Heer), Air Force (Luftwaffe), and SS (Schutzstaffel) had betrayed him. Since the German Navy had been too small to affect the war in a major way, its commander, Dönitz, became the only possible successor more or less by default.

However, on 1 May — the day after Hitler's death — Goebbels committed suicide. Dönitz thus became the sole representative of the crumbling German Reich. He appointed Finance Minister Count Ludwig Schwerin von Krosigk as "Leading Minister" (Krosigk had declined to accept the title of Chancellor) and they attempted to form a government.

That night, Dönitz made a nationwide radio address in which he spoke of Hitler's "hero's death" and announced that the war would continue "to save Germany from destruction by the advancing Bolshevik enemy." However, Dönitz knew even then that Germany's position was untenable and that the Wehrmacht was no longer capable of offering meaningful resistance. During his brief period in office, Dönitz devoted most of his efforts to ensuring the loyalty of the German armed forces and trying to ensure German troops would surrender to the British or Americans and not the Soviets. He feared vengeful Soviet reprisals against Nazi party members and high ranking officers like himself, and hoped to strike a deal with the western Allies.

The rapidly advancing Allied forces limited the Dönitz government's jurisdiction to an area around Flensburg near the Danish border, where Dönitz's headquarters were located, along with Mürwik. Accordingly his administration was referred to as the Flensburg government. The following is Dönitz's description of his new government:

Late on 1 May, Himmler attempted to make a place for himself in the Flensburg government. The following is Dönitz's description of his showdown with Himmler:

On 4 May, German forces in the Netherlands, Denmark, and northwestern Germany under Dönitz's command surrendered to Field Marshal Bernard Montgomery at the Lüneburg Heath, just southeast of Hamburg, signalling the end of World War II in northwestern Europe.

A day later, Dönitz sent Admiral Hans - Georg von Friedeburg, his successor as the commander in chief of the German Navy, to U.S. General Dwight D. Eisenhower's headquarters in Rheims, France, to negotiate a surrender to the Allies. The Chief of Staff of OKW, Colonel - General (Generaloberst) Alfred Jodl, arrived a day later. Dönitz had instructed them to draw out the negotiations for as long as possible so that German troops and refugees could surrender to the Western Powers. However, when Eisenhower let it be known he would not tolerate the Germans' stalling, Dönitz authorised Jodl to sign the instrument of unconditional surrender at 1:30 a.m. on the morning of May 7. Just over an hour later, Jodl signed the documents. The surrender documents included the phrase, "All forces under German control to cease active operations at 23:01 hours Central European Time on 8 May 1945." At Stalin's insistence, on 8 May, shortly before midnight, General Field Marshal (Generalfeldmarschall) Wilhelm Keitel repeated the signing in Berlin at Marshal Georgiy Zhukov's headquarters, with General Carl Spaatz of the USAAF as Eisenhower's representative. At the time specified, World War II in Europe ended.

On 23 May, the Dönitz government was dissolved when its members were arrested by the Allied Control Commission at Flensburg.

Despite his postwar claims, Dönitz was seen as supportive of Nazism during the war. Several naval officers described him as "closely tied to Hitler and Nazi ideology." On one occasion, he went as far as to boast about Hitler's humanity. Another event, in which he spoke to Hitler Youth in what was defined as an "inappropriate way", earned him the nickname of "Hitler Youth Dönitz." He refused to assist Albert Speer in stopping a scorched earth policy dictated by Hitler and is also noted as saying, "in comparison to Hitler we are all pip-squeaks. Anyone who believes he can do better than the Führer is stupid."

There are several antisemitic statements on the part of Dönitz known to historians. When Sweden closed its international waters to Germany, he blamed this action on their fear and dependence on "international Jewish capital." In August 1944, he declared, "I would rather eat dirt than see my grandchildren grow up in the filthy, poisonous atmosphere of Jewry."

On German Heroes' Day (12 March) 1944, Dönitz declared, without Adolf Hitler, Germany would be beset by "poison of Jewry," the country destroyed for lack of National Socialism which, as Dönitz declared, gave defiance of an uncompromising ideology. At the Nuremberg Trials, Dönitz claimed the statement about "poison of Jewry" was regarding "the endurance, the power to endure, of the people, as it was composed, could be better preserved than if there were Jewish elements in the nation." Initially he claimed, "I could imagine that it would be very difficult for the population in the towns to hold out under the strain of heavy bombing attacks if such an influence was allowed to work."

Author Eric Zillmer argues that from an ideological standpoint, Dönitz was anti - Marxist and anti - Semitic. Later, during the Nuremberg Trials, Dönitz claimed to know nothing about the extermination of Jews and declared nobody among "his men" thought about violence against Jews.

Dönitz told Leon Goldensohn, an American psychiatrist at Nuremberg, "I never had any idea of the goings-on as far as Jews were concerned. Hitler said each man should take care of his business, and mine was U-boats and the navy". To Goldensohn, Dönitz also spoke of his support for Admiral Bernhard Rogge, who was of Jewish descent, when the Nazi Party began to persecute the admiral.

Following the war, Dönitz was held as a prisoner of war by the Allies. He was indicted as a major war criminal at the Nuremberg Trials on three counts: (1) conspiracy to commit crimes against peace, war crimes, and crimes against humanity; (2) Planning, initiating and waging wars of aggression; and (3) crimes against the laws of war. Dönitz was found not guilty on count (1) of the indictment, but guilty on counts (2) and (3).

During the trial, Gustave Gilbert, an American Army psychologist, was allowed to examine the Nazi leaders who were tried at Nuremberg for war crimes. Among other tests, a German version of the Wechsler - Bellevue IQ test was administered. Dönitz scored 138, the third highest among the Nazi leaders tested.

Dönitz disputed the propriety of his trial at Nuremberg, commenting on count (2) that "One of the 'accusations' that made me guilty during this trial was that I met and planned the course of the war with Hitler; now I ask them in heaven’s name, how could an admiral do otherwise with his country's head of state in a time of war?" Numerous (over 100) Senior Allied officers also sent letters to Dönitz conveying their disappointment over the fairness and verdict of his trial.

At the trial Dönitz was charged with:

• Waging unrestricted submarine warfare against neutral shipping.
• Permitting Hitler's Commando Order of 18 October 1942 to remain in full force when he became commander - in - chief of the Navy, and to that extent responsibility for that crime. His defence was that the Order excluded men captured in naval warfare, and that the order had not been acted upon by any men under his command.
• That knowing that 12,000 involuntary foreign workers were working in the shipyards, he did nothing to stop it.
• Advice in 1945 when Hitler asked Dönitz whether the Geneva Convention should be denounced. Hitler's motives were twofold. The first was that reprisals could be taken against Western Allied prisoners of war and second it would deter German forces from surrendering to the Western Allies (as was happening on the Eastern front where the Geneva Convention was in abeyance). Instead of arguing that the conventions should never be denounced, Dönitz suggested that it was not currently expedient to do so, so the court found against him on this issue; but as the Convention was not denounced by Germany, and British prisoners in camps under Dönitz's jurisdiction were treated strictly according to the Convention, the Court considered these mitigating circumstances.

Among the war crimes charges, Dönitz was accused of waging unrestricted submarine warfare for issuing War Order No. 154 in 1939, and another similar order after the Laconia incident in 1942, not to rescue survivors from ships attacked by submarine. By issuing these two orders, he was found guilty of causing Germany to be in breach of the Second London Naval Treaty of 1936. However, as evidence of similar conduct by the Allies was presented at his trial, and with the help of his lawyer Otto Kranzbühler, his sentence was not assessed on the grounds of this breach of international law.

On the specific war crimes charge of ordering unrestricted submarine warfare Dönitz was found "[not] guilty for his conduct of submarine warfare against British armed merchant ships", because they were often armed and equipped with radios which they used to notify the Admiralty of attack but the judges found that "Dönitz is charged with waging unrestricted submarine warfare contrary to the Naval Protocol of 1936 to which Germany acceded, and which reaffirmed the rules of submarine warfare laid down in the London Naval Agreement of 1930... The order of Dönitz to sink neutral ships without warning when found within these zones was, therefore, in the opinion of the Tribunal, violation of the Protocol... The orders, then, prove Dönitz is guilty of a violation of the Protocol... the sentence of Dönitz is not assessed on the ground of his breaches of the international law of submarine warfare."

His sentence on unrestricted submarine warfare was not assessed, because of similar actions by the Allies: in particular, the British Admiralty on 8 May 1940 had ordered that all vessels in the Skagerrak should be sunk on sight; and Admiral Chester Nimitz, wartime commander - in - chief of the U.S. Pacific Fleet, stated that the U.S. Navy had waged unrestricted submarine warfare in the Pacific from the day the U.S. entered the war. Thus although Dönitz's was found guilty of waging unrestricted submarine warfare against unarmed neutral shipping by ordering all ships in designated areas in international waters to be sunk without warning, no additional prison time was added to his sentence for this crime.

Dönitz was imprisoned for 10 years in Spandau Prison in what was then West Berlin.

Dönitz was released on 1 October 1956, and he retired to the small village of Aumühle in Schleswig - Holstein in northern West Germany. There he worked on two books. His memoirs, Zehn Jahre, Zwanzig Tage (Memoirs: Ten Years and Twenty Days), appeared in Germany in 1958 and became available in an English translation the following year. This book recounted Dönitz's experiences as U-boat commander (10 years) and President of Germany (20 days). In it, Dönitz explains the Nazi regime as a product of its time, but argues he was not a politician and thus not morally responsible for much of the regime's crimes. He likewise criticizes dictatorship as a fundamentally flawed form of government and blames it for much of the Nazi era's failings.

Admiral Dönitz's second book, Mein wechselvolles Leben (My Ever-Changing Life) is less known, perhaps because it deals with the events of his life before 1934. This book was first published in 1968, and a new edition was released in 1998 with the revised title Mein soldatisches Leben (My Life as a Soldier).

Late in his life, Dönitz made every attempt to answer correspondence and autograph postcards for others. Dönitz was unrepentant regarding his role in World War II since he firmly believed that no one will respect anyone who compromises with his belief or duty towards his nation in any way, whether his betrayal was small or big. Of this conviction, Dönitz writes (commenting on Himmler's peace negotiations):

The betrayer of military secrets is a pariah, despised by every man and every nation. Even the enemy whom he serves has no respect for him, but merely uses him. Any nation which is not uncompromisingly unanimous in its condemnation of this type of treachery is undermining the very foundations of its own state, whatever its form of government may be.

Dönitz lived out the rest of his life in relative obscurity in Aumühle, occasionally corresponding with American collectors of German Naval history, and died there of a heart attack on 24 December 1980. As the last German officer with the rank of Grand Admiral, he was honoured by many former servicemen and foreign naval officers who came to pay their respects at his funeral on 6 January 1981. However, he had received only the pension pay of a captain because the West German government ruled all of his advances in rank after that had been because of Hitler. He was buried in Waldfriedhof Cemetery in Aumühle without military honors, and soldiers were not allowed to wear uniforms to the funeral. However a number of German naval officers disobeyed this order and were joined by members of the Royal Navy, such as the senior chaplain the Rev Dr John Cameron, in full dress uniform. Also in attendance were over one hundred holders of the Knight's Cross.

On 27 May 1916 Dönitz married a nurse named Ingeborg Weber, the daughter of a German general. They had three children whom they raised as Protestant (Evangelical) Christians, viz., daughter Ursula (b. 1917) and sons Klaus (b. 1920) and Peter (b. 1922). Both sons were killed during the Second World War. The younger son, Peter, was a watch officer on U-954 and was killed on 19 May 1943, when his boat was sunk in the North Atlantic with all hands. After this loss, the older brother, Klaus, was allowed to leave combat duty and began studying to be a naval doctor. Klaus was killed on 13 May 1944 while taking part in an action against his orders. Klaus convinced his friends to let him go on the torpedo boat S-141 for a raid on HMS Selsey off the coast of England on his twenty-fourth birthday. The boat was destroyed and Klaus died, though six others were rescued. In 1937 Karl Dönitz's daughter Ursula married the U-boat commander and Knight's Cross of the Iron Cross recipient Günther Hessler.