As a boy he was instructed in the military art by his father, an officer of the Corps de genie, and at the age of seventeen he entered the army, serving successively in the staff, the engineers and the prince de Lambesq's dragoons. In 1780 he went to North America with Rochambeau, and on his return, having attained the rank of colonel, he was employed in various staff posts and in a military mission to Prussia. During the Revolution, as chief of staff of the Versailles national guard, he protected the aunts of Louis XVI from popular violence, and aided their escape (1791).
In the war of 1792 he was at once made chief of staff to Marshal Lückner, and he bore a distinguished part in the Argonne campaign of Dumouriez and Kellermann. He served with great credit in the Vendéan War of 1793-95, and was in the next year made a general of division and chief of staff (Major-Général) to the army of Italy, which Bonaparte had recently been appointed to command. His power of work, accuracy and quick comprehension, combined with his long and varied experience and his complete mastery of detail, made him the ideal chief of staff to a great soldier; and in this capacity he was Napoleon's most valued assistant for the rest of his career.
He accompanied Napoleon throughout the brilliant campaign of 1796, and was left in charge of the army after the peace of Campo Formio. In this post he organized the Roman republic (1798), after which he joined his chief in Egypt, serving there until Napoleon's return. He assisted in the coup d'état of 18th Brumaire, afterwards becoming minister of war for a time. In the campaign of Marengo he was the nominal head of the Army of Reserve, but the first consul accompanied the army and Berthier acted in reality, as always, as chief of staff to Napoleon. At the close of the campaign he was employed in civil and diplomatic business.
When Napoleon became emperor, Berthier was at once made a marshal of the empire. He took part in the campaigns of Austerhtz, Jena and Friedland, and was created duke of Valengin in 1806, sovereign prince of Neuchâtel in the same year and vice-constable of the empire in 1807. In 1808 he served in the Peninsula, and in 1809 in the Austrian War, after which he was given the title of prince of Wagram. Berthier married a niece of the king of Bavaria. He was with Napoleon in Russia in 1812, Germany in 1813, and France in 1814, fulfilling, till the fall of the empire, the functions of "major-general" of the Grande Armée.
He abandoned Napoleon to make his peace with Louis XVIII in 1814, and accompanied the king in his solemn entry into Paris. During Napoleon's captivity in Elba, Berthier, whom he informed of his projects, was much perplexed as to his future course, and, being unwilling to commit himself, fell under the suspicion both of his old leader and of Louis XVIII. On Napoleon's return he withdrew to Bamberg, where he later died.
The manner of his death is uncertain; according to some accounts he was assassinated by members of a secret society, others say that, maddened by the sight of Russian troops marching to invade France, he threw himself from his window and was killed. Berthier was not a great commander. When he was in temporary command in 1809 the French army in Bavaria underwent a series of reverses. Whatever merit as a general he may have possessed was completely overshadowed by the genius of his master. But his title to fame is that he understood and carried out that master's directions to the minutest detail.