He was a captain when, in 1791, he embraced the principles of the French Revolution. Moncey won great distinction in the campaigns of 1793 and 1794 on the Spanish frontier, rising from the command of a battalion to the command in chief of the Army of the Western Pyrenees in a few months, and his successful operations were largely instrumental in compelling the Spanish government to make peace. After this he was employed in the highest commands until 1799, when the government, suspecting him of Royalist views, dismissed him. From 1801–1815 he was inspector general of the police.
But the coup d'état of 18 Brumaire in 1799 brought him back to the active list, and in Napoleon's Italian campaign of 1800 he led a corps from Switzerland into Italy, surmounting all the difficulties of bringing horses and guns over the then formidable pass of St Gothard. In 1801 Napoleon made him inspector-general of gendarmerie, and on the assumption of the imperial title created him a marshal of France. In 1805 Moncey received the grand cordon of the legion of honor.
In 1808 he was made duke of Conegliano, or rather duc; for it was a duché grand-fief, a rare hereditary -but nominal- honor created in Napoleon's own kingdom of Italy (extinguished with his own death in 1842). That year, the first of the Peninsular War, Moncey was sent to Spain in command of an army corps. He signalized himself by his victorious advance on Valencia, the effect of which was however, destroyed by the disaster to Dupont at Bailén, and took a leading part in the emperor's campaign on the Ebro and in the Second Siege of Saragossa in 1809.
He refused to serve in the invasion of Russia, and therefore had no share in the campaign of the Grande Armée in 1812 and 1813. When, however, France was invaded in 1814, Marshal Moncey reappeared in the field and fought the last battle for Paris on the heights of Montmartre and at the barrier of Clichy.
In 1814 he supported Louis XVIII and was named Peer of France. He remained neutral during Napoleon's return, the 'Hundred Days', feeling himself bound to Louis XVIII by his engagements as a Peer of France, but after Waterloo he was punished for refusing to take part in the court martial of Marshal Ney by imprisonment and the loss of his marshalate.
The King returned his titles in 1816, and he re-entered the chamber of peers three years later. He continued his military career- his last active service was as commander of an army corps in the short war with Spain in 1823. In 1833 he became governor of the prestigious Hôtel des Invalides (for veterans, in Paris). Present at the return of Napoleon's ashes in December 1840, he said after the ceremony, "Now, let's go home to die".
He married Charlotte Prospère Remillet (1761 – 1842), by whom he had female issue which, however, kept using his title until its extinction in 1901.