Canaris was born in Aplerbeck (now a part of Dortmund) in Westphalia, the son of wealthy industrialist Carl Canaris and his wife Auguste (née Popp). Until 1938 Canaris believed that his family was related to the Greek admiral, freedom fighter and politician Constantine Kanaris, which influenced his decision to join the navy. While on a visit to Corfu he was given a portrait of the Greek hero, which he always kept in his office. In 1938, however, research showed that his family was of North Italian descent, originally called Canarisi, and had lived in Germany since the 17th century. His grandfather had converted from Catholicism to Lutheranism.

In 1905, aged seventeen, Canaris joined the German Imperial Navy and by the outbreak of World War I was serving on board the SMS Dresden as intelligence officer. This cruiser was the only ship that managed to evade the British Fleet for a prolonged period during the Battle of the Falkland Islands in December 1914, largely due to his excellent deception tactics. Whilst anchored in Cumberland Bay, Robinson Crusoe Island, the Dresden was trapped and forced to scuttle after fighting a battle there with the British. Most of the crew became prisoners in Chile in March 1915, but Canaris escaped in August 1915, using his fluency in Spanish; with the aid of some German merchants he returned to Germany in October 1917 via, among other countries, Great Britain.

He was then given intelligence work and sent to Spain, where he survived a British assassination attempt. Returning to active service, he ended the war as a celebrated U-boat commander from late 1917 in the Mediterranean, credited with eighteen sinkings. By that time he was an enemy on the top of the list of the MI6 secret service in the UK. He spoke English fluently (as well as four other foreign languages) and as a naval officer of the old school, he respected Great Britain's Royal Navy despite the rivalry between the two nations.

During the German Revolution of 1918 – 19, Canaris helped organize the formation of vigilant forces in order to suppress the revolutionary movements. He was also a member of the military court that tried (and mostly acquitted) those involved in the assassination of Karl Liebknecht and Rosa Luxemburg. He was appointed to the adjutancy of defence minister Gustav Noske.

In 1919, Canaris married Erika Waag, also the child of an industrialist. They had two daughters, Eva and Brigitte.

Canaris remained in the military after the war, first as a member of the Freikorps and then as part of the Reichsmarine. He was promoted rapidly, becoming a Captain in 1931, the Executive Officer of the cruiser Berlin and then the Commanding Officer of the battleship Schlesien. At this time, he became involved in intelligence work again. He made a series of contacts with high ranking German officers, politicians and industrialists for the purpose of creating order in German politics. During his Freikorps period, he was on intimate terms with the people such as Horst von Pflugk - Hartung who were accused of political assassinations of leaders of the left, and was even accused himself, although later acquitted, of being involved in the assassinations and other crimes (such as his alleged involvement in Rosa Luxembourg's "trial"). During the 1930 – 33 period, Canaris was following a course quite parallel to the one followed by the future Nazi party leaders although never a party member himself. Indirectly, though, he promoted the forces that later became part of the Nazi power structure.

After Adolf Hitler's rise to power in 1933, Canaris was made head of the Abwehr, Germany's official military intelligence agency, on 1 January 1935. Later that year, he was promoted to Rear Admiral. During the period 1935 – 36, he made contacts in Spain to organize a German spy network there, due to his excellent Spanish. He was the moving force behind the decision that sided Germany with Francisco Franco during the Spanish Civil War, despite Hitler's initial hesitation to get involved in such an adventure.

In 1937 he was still a supporter of Hitler, considering him to be the only solution against communism and a hope for the national revival of Germany. By 1938, however, he had realised that Hitler's policies and plans would lead Germany to disaster and secretly began to work against the régime. His personal style as a gentleman was incompatible with the thuggish behaviour of most of the Nazi party members. A letter from a Spanish contact of his has been preserved and unambiguously confirms his opposition to the Nazi regime.

He tried to hinder Hitler's attempts to absorb Czechoslovakia and he also advised Franco not to permit German passage through Spain for the purposes of capturing Gibraltar. Arguments used by Franco to counter Hitler's demands for German access to Spanish territory were influenced directly by Canaris, who met with a number of his top advisors. Additionally, a significant sum of money had been deposited by the British in Swiss accounts for Franco and his generals to maintain their neutrality.
He also became involved in two abortive plots to assassinate Hitler, first in 1938 and again in 1939. During the 1938 crisis over Czechoslovakia that culminated in the Munich Agreement, Canaris was together with the Army Chief of Staff, General Ludwig Beck and the Foreign Office’s State Secretary Ernst von Weizsäcker. Canaris was a leader of the "anti-war" group in the German government, which was determined to avoid a war in 1938 that it felt Germany would lose.

This group was not necessarily committed to the overthrow of the regime, but was loosely allied to another, more radical group, the "anti-Nazi" fraction centered around Colonel Hans Oster and Hans Bernd Gisevius, which wanted to use the crisis as an excuse for executing a putsch to overthrow the Nazi regime.

His most audacious attempt was in planning, with Ewald von Kleist - Schmenzin, to capture and eliminate Hitler and the entire Nazi party before the invasion of Czechoslovakia. At this particular moment, von Kleist visited Britain secretly and discussed the situation with British MI6 and some high ranking politicians. There, the name of Canaris became widely known as the executive hand of von Kleist in the event of an anti-Nazi plot. The high ranking German military leaders believed that if Hitler invaded Czechoslovakia, or any other country, then Britain would declare war on Germany. MI6 was of the same opinion. The British declaration of war would have given the General Staff, in their belief, both the pretext and support for an overthrow of Hitler.

The British reaction, however, to Hitler's demands on the Sudetenland was more cautious. At a meeting with Hitler in Munich, British Prime Minister Neville Chamberlain, (1869 – November 1940), and French Prime Minister Édouard Daladier chose diplomacy over war. Munich was a severe disappointment for Kleist and Canaris. It gave Hitler's international reputation an important boost for two reasons: one, he was able to play the part of a man of reason and compromise; and two, he could boast that his predictions that Great Britain and France would not respond with war had proven to be correct. There are claims that Canaris, who was extremely shocked by this 'dishonest and stupid decision' (his own words), decided to be cautious and wait for a better time to act against Hitler.

In January 1939, Canaris manufactured the "Dutch War Scare", which gripped the British government. By 23 January 1939 the British government received information to the effect that Germany intended to invade the Netherlands in February 1939 with the aim of using Dutch air fields to launch strategic bombing offensive intended to achieve a "knock - out" blow against Britain by razing British cities to the ground. All this information was false, and it was intended by Canaris to achieve a change in British foreign policy. In this, Canaris was successful, and the "Dutch War Scare" played a major role in causing Chamberlain to make the "continental commitment" (i.e. sending a large British ground force to the defence of France) in February 1939.

Nevertheless, it appears likely that MI6 maintained contact with Canaris even after the Munich Agreement signed on 30 September 1938. When Winston Churchill came to power after the resignation of Chamberlain in May 1940, Canaris' hopes were renewed, given the new Prime Minister's strong position against Hitler.

In the meantime, Reinhard Heydrich, previously a naval cadet who had served under Canaris and was at the time the Sicherheitsdienst (SD) leader, despite being his protégé, friend and neighbour, became his rival. Presumably, the Canaris posting in Abwehr had the secret approval of the dynamic Heydrich, who preferred him to his predecessor, Commander Pfatz, who was not in line with the Nazi party members. Heydrich wanted a controllable Abwehr and was keeping a close eye on Canaris. Canaris appeared outwardly to side with his friend Heydrich, but only in order to give Abwehr a chance to grow and become a considerable force. In Bassett's account, Canaris was deeply frustrated by a briefing from Hitler before the attack on Poland. During the briefing, he was informed about a series of exterminations that had been ordered and which Canaris was required to take notes on. These notes, the book confirms, were sent to MI6. After the outbreak of war between Germany and Poland, in September 1939, Canaris visited the front and witnessed examples of the war crimes committed by the SS Einsatzgruppen. Among these were the burning of the synagogue in Będzin, where the town's Jewish residents were burned to death. He also received reports from Abwehr agents about many other incidents of mass murder throughout Poland. Canaris kept detailed records of these atrocities in his personal diary which he entrusted to Werner Schrader, one of his subordinates and fellow resistance member.

After hearing reports of and witnessing massacres in Poland, Canaris on 12 September 1940 travelled to Hitler's headquarters train, at the time in Upper Silesia, to register his objection to the atrocities; prior to reaching Hitler he encountered General Wilhelm Keitel whom he informed: "I have information that mass executions are being planned in Poland, and that members of the Polish nobility and the Roman Catholic bishops and priests have been singled out for extermination." Keitel admonished Canaris to go no further with his protest as the detailed plan of atrocities came directly from Hitler, himself.

Shocked by these incidents, Canaris began working more actively, at increasing risk, to overthrow Hitler's régime, although he cooperated with the SD to create a decoy. This made it possible for him to pose as a trusted man for some time. He was promoted to full Admiral in January 1940. With his subordinate Erwin Lahousen, he formed a circle of like minded Wehrmacht officers, many of whom would be executed or forced to commit suicide after the failure of the 20 July Plot.

It has been speculated that there was contact with British intelligence during this time, despite the war between the two countries. It is thought that during the invasion of Russia, Canaris received a detailed report of all the enemy positions that was known only to the British. The head of MI6, Stewart Menzies, who shared Canaris’s strong anti-communist beliefs, praised Canaris’s courage and bravery at the end of the war. Reinhard Heydrich and Heinrich Himmler, however, investigated in detail the sources of Canaris's information on Operation Barbarossa, arriving at the conclusion that there had indeed been contact between him and the British.

After 1942, Canaris visited Spain frequently and was probably in contact with British agents from Gibraltar. In 1943, while in occupied France, Canaris is said to have made contact with British agents: he was conducted blindfolded to the Convent of the Nuns of the Passion of our Blessed Lord, 127 Rue de la Santé, where he met the local head of the British Intelligence Services, code name "Jade Amicol", in reality Colonel Claude Olivier. Canaris wanted to know the terms for peace if Germany got rid of Hitler. Churchill's reply, sent to him two weeks later, was simple: "Unconditional surrender".

During Heydrich's posting in Prague, a serious incident put him and Canaris in open conflict. A Czech agent — Paul Thümmel — was arrested by Heydrich, but Canaris intervened to save him, claiming he was a double agent actually working for Abwehr. Heydrich suspected that Thümmel was actually Canaris's MI6 contact. Heydrich requested that Canaris put the Abwehr under SD and SS control. Canaris appeared to retreat and handled the situation diplomatically, but there was no immediate effect on the Abwehr for the time being. In fact, Canaris had established another two links with MI6 — one via Zurich, and the other via Spain and Gibraltar. It is also possible that Vatican contacts provided a third route to his British counterparts.

Canaris also intervened to save a number of victims of Nazi persecution, including saving Jews, some by getting them to Spain. Many such people were given token training as Abwehr "agents" and then issued papers allowing them to leave Germany. One notable person he is said to have assisted was the then Lubavitcher Rebbe in Warsaw, Rabbi Yosef Yitzchok Schneersohn. This has led Chabad Lubavitch to campaign for his recognition as a Righteous Gentile by the Yad VaShem holocaust memorial.

The assassination of Reinhard Heydrich in Prague, organized by MI6, was done in part to preserve Canaris in his important position.

Colonel Wessel von Freytag - Loringhoven's son Niki, testifying in Munich in 1972 and in recent revelations, reports that Canaris was involved in the foiling of Hitler's plot to kidnap Pope Pius XII. Colonel Freytag - Loringhoven was a subordinate of Canaris, and his son, Niki von Freytag - Loringhoven, reported that within days of the arrest of Benito Mussolini as ordered by King Victor Emmanuel III, the Führer commanded the Reichssicherheitshauptamt (the Third Reich’s Security Headquarters) to retaliate against the Italians via the kidnapping or murder of Pius XII and King Victor Emmanuel.

The colonel’s son, Niki Freytag - Loringhoven, now 72, recently came forward to reveal new details about the plan, reporting that on 29 and 30 July 1943 his father and Erwin von Lahousen, who were employed in the section of German intelligence dealing mainly with sabotage, attended a meeting in Venice where Canaris informed the Italian General, Cesare Amè, of the plot. General Amè relayed the news which allowed the plot to be foiled. The Italian paper, Avvenire, maintains that the younger Freytag von Loringhoven’s accounts comport with the Von Lahousen’s Nuremberg war crimes trials deposition.

The evidence that he was playing a double game grew, and at the insistence of Heinrich Himmler, who had suspected him for a long time, Hitler dismissed Canaris from the Abwehr in February 1944, replacing him with Walter Schellenberg and merging most of the Abwehr with the Sicherheitsdienst (SD). Some weeks later, Canaris was put under house arrest, preventing him from taking part directly in the 20 July Plot, 1944, to assassinate Hitler.

However, just after the Stalingrad disaster, Canaris had already planned a 'coup' against the entire Nazi regime in which many Nazi officials would be accused for known crimes, while Hitler would be arrested as an insane person based on his exposure to poison gas in World War I, then imprisoned for life. After the 20 July Plot, Canaris's long time rival, SS leader Heinrich Himmler discovered that one of the officers involved in the plot, a friend of Canaris who had committed suicide, had kept the plot details in a metal box. The investigations also revealed that a number of other assassination plots (possibly another 10 or 15) had been activated but had failed and were covered up at the last minute. Most people who participated in these plots were people Canaris knew well. In the aftermath of the attempt on Hitler's life, the Gestapo found no direct evidence tying Canaris to the plot, but his close association to many of the conspirators that were arrested was enough to seal his fate.

Himmler kept Canaris alive for some time because he planned to use him secretly as a future contact with the British in order to come to an agreement to end the war with himself as the leader of Germany. Hitler also wanted to keep him alive in order to get the names of additional conspirators. When Himmler's plan failed to materialize, he received the approval of Hitler to send Canaris to an SS drumhead court martial presided over by Otto Thorbeck with Walter Huppenkothen as prosecutor that sentenced him to death.

Together with his deputy General Hans Oster, military jurist General Karl Sack, theologian Rev. Dietrich Bonhoeffer and Ludwig Gehre, Canaris was humiliated before witnesses and then executed on 9 April 1945, in the Flossenbürg concentration camp, just weeks before the end of the war. He was led to the gallows barefoot and naked. Just before his execution he tapped out a message in morse code, which was heard by another prisoner, claiming he acted for the good of Germany and denying he was a traitor. At the time of his execution, Canaris had been decorated with the Iron Cross First and Second Class, the Silver German Cross, the Cross of Honour and the Wehrmacht Twelve and Twenty-Five Year Long Service Ribbons.

Erwin von Lahousen and Hans Bernd Gisevius, two of Canaris' main subordinates, survived the war and testified during the Nuremberg Trials about Canaris' courage in opposing Hitler. Lahousen recalled a conversation between Canaris and General Wilhelm Keitel in which Canaris warned Keitel that the German military would be held responsible for the atrocities in Poland. Keitel responded that they had been ordered by Hitler. Keitel, who also survived the war, was found guilty of war crimes at Nuremberg and hanged.

• The 1954 movie Canaris is based on his biography.
• In the 1961 novel, 'Es muss nicht immer Kaviar sein' by German author Johannes Mario Simmel Canaris is the primary benefactor of agent Thomas Lieven during his time as German Agent in World War II. The novel is claimed by the author to be authentic.
• In the 1970 Colin Forbes novel The Heights of Zervos, Canaris is mentioned along with the Abwehr.
• In the 1976 film The Eagle Has Landed, Canaris was played by actor Anthony Quayle.
• In the Frederick Forsyth novel The Odessa File, set in the mid 1960s, the hero infiltrates the organisation of former SS members by claiming to have commanded, as a 19 year old sergeant, the firing squad which executed Canaris. This is not in fact how Canaris was executed, which was by hanging.
• In the 1980 Brian Garfield novel The Paladin, Canaris is visited by an agent acting for Churchill. It is apparent that in this book, Canaris is acting as a knowing conduit for British misinformation.
• In the 1996 Daniel Silva novel The Unlikely Spy, Canaris is the head of the Abwehr who initiated the infiltration of SHAEF to discover its invasion plans of Normandy.