Weygand was born in Brussels of unknown parents. He was long suspected of being the illegitimate son of either Empress Carlota of Mexico (by General Alfred van der Smissen); or of her brother Leopold II, King of the Belgians, and Leopold's Polish mistress. Van der Smissen always seemed a likely candidate for Weygand's father because of the striking resemblance between the two men. In 2003, the French journalist Dominique Paoli claimed to have found evidence that Weygand's father was indeed van der Smissen, but the mother was Mélanie Zuchy-Metternich, lady-in-waiting to Carlota (and daughter of Prince Metternich, Austrian Chancellor). Paoli further claimed that Weygand had been born in mid 1865, not January 1867 as is generally claimed. Regardless, throughout his life Weygand maintained he did not know his true parentage. While an infant he was sent to Marseille to be raised by a widow named Virginie Saget, whom he originally took to be his mother. At age 6 he was transferred to the household of David Cohen de Léon, a financier of Sephardic origins who was a friend of Leopold II. Upon reaching adulthood, Weygand was legally acknowledged as a son by Francois-Joseph Weygand, an accountant in the employ of M. Cohen de Léon, thereby granting him French citizenship. In his memoirs he says little about his youth, devoting to it only 4 pages out of 651. He mentions the gouvernante and the aumônier of his college, who instilled in him a strong Roman Catholic faith. His memoirs essentially begin with his entry into the preparatory class of Saint-Cyr Military School in Paris, as if he had wished to disregard his connection with Mme. Saget and M. Cohen de Leon.

He was admitted to the École Spéciale Militaire de Saint-Cyr, under the name of "Maxime de Nimal" as a foreign cadet (Belgian). Successfully graduating in 1887, he was posted to a cavalry regiment. After changing his name to Weygand and receiving French nationality, he became an instructor at Saumur. During the Dreyfus affair, he was one of the most antidreyfusard officers of his regiment, supporting the widow of Colonel Henry, who had committed suicide after the discovery of the falsification of the charges against Captain Dreyfus.

Once promoted to Captain, Weygand chose not to attempt the difficult preparation to the Advanced War College ('Haute Ecole de Guerre') because of his desire, he said, to keep contact with the troops. This did not prevent him from later becoming an instructor at the aristocratic Cavalry School at Saumur.

Weygand spent the war of 1914-18 as a Staff Officer. At the outbreak, he satisfied his taste for contact with the troops while spending 26 days with the 5ème Hussards. On 28 August, he became a Lieutenant-Colonel on the staff of General Ferdinand Foch. He was promoted to Brigadier General in 1916 and Général de Division (equivalent to the Anglophone rank of Major General) in 1918, serving in the Supreme War Council from 1917. He remained on Foch's staff when his patron was appointed Supreme Allied Commander in the spring of 1918, and was Foch's right-hand man throughout his victory at the Second Marne (for which Foch was promoted to Marshal of France) in the late summer and until the end of the war.

In 1918 Weygand served on the armistice negotiations, and it was Weygand who read out the armistice conditions to the Germans at Compiègne, in the twice infamous railway carriage. He can be spotted in photographs of the armistice delegates, and also standing behind Foch's shoulder at Pétain's investiture as Marshal of France at the end of 1918.

Weygand was briefly sent to Poland as head of the French military mission in 1920 during the Polish-Soviet War. The mission also included French diplomat Jean Jules Jusserand and the British diplomat Lord Edgar Vincent D'Abernon. It achieved little; indeed, the crucial Battle of Warsaw was fought and won by the Poles before the mission could return and make its report. Subsequently, for many years, the myth that the timely arrival of Allied forces saved Poland was begun, a myth in which Weygand occupies the central role.

Weygand travelled to Warsaw in the expectation of assuming command of the Polish army, yet he met with a very disappointing reception. His first meeting with Piłsudski on 24 July started on the wrong foot, as he had no answer to Piłsudski's opening question, "How many divisions do you bring?" Weygand had no divisions to offer. On 27 July, he was installed as adviser to the Polish Chief of Staff, Rozwadowski, but their cooperation was poor. He was surrounded by officers who regarded him as an interloper and who deliberately spoke in Polish, depriving him not only of a part in their discussions but even of the news from the front. His suggestions for the organization of Poland's defence were systematically rejected. At the end of July he proposed that the Poles hold the line of the Western Bug; a week later he proposed a purely defensive posture along the Vistula. Neither plan was accepted. One of his few contributions was to insist that a system of written staff orders should replace the existing haphazard system of orders passed by word of mouth. He was of special assistance to General Władysław Sikorski, to whom he expounded the advantages of the River Wkra. But on the whole he was quite out of his element, a man trained to give orders yet placed among people without the inclination to obey, a proponent of defence in the company of enthusiasts for the attack. On 18 August, when he met Piłsudski again he was told nothing of the great victory, but was "regaled instead with a Jewish tale". It offended his dignity as a "représentant de la France" and he threatened to leave. Indeed there was nothing to do but leave. The battle was won; armistice negotiations were beginning; the crisis had passed. He urged D'Abernon and Jusserand to pack their bags and make as decent an exit as possible. He was depressed by his failure and dismayed by Poland's disregard for the Entente. On the station at Warsaw on 25 August he was consoled by the award of the medal, the Virtuti Militari; at Kraków on the 26th he was dined by the mayor and corporation; at Paris on the 28th he was cheered by crowds lining the platform of the Gare de l'Est, kissed on both cheeks by the Premier Alexandre Millerand and presented with the grand-croix de la légion d'honneur. He could not understand what had happened and has admitted in his memoirs that "the victory was Polish, the plan was Polish, the army was Polish". He was the first uncomprehending victim, as well as the chief beneficiary, of a legend already in circulation that he, Weygand, was the victor of Warsaw. This legend persisted for more than forty years even in academic circles.

Weygand was unemployed for a time after the military mission to Poland, but in 1923 he was made commander-in-chief of the Levant, the French mandate in Lebanon and Syria. He was then appointed High Commissioner of Syria the next year, a position he also only kept for a year. Weygand returned to France in 1925, when he became director of the Center for Higher Military Studies, a position he had for five years. In 1931 he was appointed Chief of Staff of the French Army, Vice President of the Supreme War Council and Inspector of the Army, and was elected a member of the Académie française (seat #35). He remained in the positions, except Inspector of the Army, until his retirement in 1935 at 68. He was recalled for active service in August 1939 by Prime Minister Édouard Daladier and appointed commander-in-chief for the Orient Theatre of Operation.

By late May 1940 the military disaster in France after the German invasion was such that the Supreme Commander, Maurice Gamelin, was dismissed, and Weygand recalled from Syria to replace him. Weygand arrived on May 17 and started by cancelling the side counter offensive ordered by Gamelin, to cut off the enemy armoured columns which had punched through the French front at the Ardennes. Thus he lost two crucial days before finally adopting the solution, however obvious, of his predecessor. But it was by then a failed manoeuvre, because during the 48 lost hours, the German infantry had caught up behind their tanks in the breakthrough and had consolidated their gains. The situation was not helped by Britain's refusal to send more than four squadrons of fighter aircraft to France, instead of the twenty which Weygand requested. More problematically, General Gort's Expeditionary Force broke promises to assist in the counter attack and instead fled to Dunkirk where it would soon be evacuated. Weygand then oversaw the creation of the Weygand line, an early application of the Hedgehog tactic; however, by this point the situation was untenable, with most of the Allied forces trapped in Belgium. Weygand complained that he had been summoned two weeks too late to halt the invasion. After some further vain attempts to contain the enemy offensive, he then joined in seeking an armistice and cooperation with the German occupiers.

In June, Weygand was appointed by Petain to the Bordeaux-Vichy cabinet as Minister for National Defence for three months (June to September 1940), and then Delegate-General to the North African colonies. While there he convinced the young officers, tempted to resistance, of the justice of the armistice, by letting them hope for a later resumption of combat. He also deported opponents to concentration camps in Southern Algeria and Morocco. There, he locked up, with the complicity of Admiral Abrial, adversaries of the Vichy regime (Gaullists, Freemasons, communists, etc.), the foreign volunteers of Légion Etrangère, foreign refugees without employment (but legally admitted into France), etc. Finally, he applied Vichy's racist laws against Jews very harshly. With the complicity of the Recteur (University chancellor) G. Hardy, Weygand instituted, on his own authority, by a mere "note de service n°343QJ" of 30 September 1941, a school "numerus clausus" (quota), driving out from the colleges and from the primary schools most of the Jewish pupils, including small children aged 5 to 11. Weygand did this without any decree of Marshal Philippe Pétain's, "by analogy," he said, "to the law about Higher Education".

Weygand acquired a reputation as an opponent of collaboration when he protested, in Vichy, against the Protocols of Paris of 28 May 1941 signed by Admiral Darlan, agreements which granted bases to the Axis in Aleppo (Syria), Bizerte (Tunisia), and Dakar (Senegal) and envisaged an extensive military collaboration with Axis forces in the event of Allied countermeasures. As Simon Kitson demonstrated in his book The Hunt for Nazi Spies, Weygand remained outspoken in his criticism of Germany. Nevertheless, the Weygand General Delegation (4th Office) collaborated with Germany by delivering to Rommel's Afrika Korps 1200 French trucks and other French army vehicles (Dankworth contract of 1941), as well as heavy artillery pieces accompanied with 1000 shells per gun.

Weygand was apparently favorable to collaboration with Germany, but with discretion. Additionally, when he opposed German bases in Africa, he did not intend to be neutral or to help the Allied camp, rather he only sought to prevent France from losing prestige with the natives and keep its colonial empire. Nevertheless, since Adolf Hitler demanded full unconditional collaboration, he pressured the Vichy government to obtain the dismissal and recall of Weygand in November 1941. One year later, in November 1942, following the Allied invasion of North Africa, Weygand was arrested. He remained in confinement in Germany and then in the Itter Castle in North Tyrol with General Gamelin and few other French Third Republic personalities until May 1945, when he fell into the hands of the Americans. After returning to France, he was held as a collaborator at the Val-de-Grâce but was released in May 1946 and cleared in 1948. He died in Paris at the age of 98.