In 1791 he obtained the rank of captain in a volunteer regiment, and was ever afterwards engaged in the active duties of his profession. Under Pichegru, and Moreau, and Massena, on the Rhine and in Switzerland, he fought up his way to the command of a division. He was no less a favourite with Buonaparte, who rewarded him, for the zeal with which he seized on Hanover at the breaking of the peace of Amiens, with a marshal's truncheon.

In 1805-6-7 Marshal Mortier added greatly to his reputation; but tarnished his laurels by the rigour with which he obeyed the arbitrary injunctions of Napoleon, in the last of these years, at Hamburg. From this plundered and oppressed city he returned to the grand army, and continued his services until the campaign closed on the woody plains of Friedland.

Mortier, now Duke of Treviso, was next summoned to Spain, where he met with no success. In the still more disastrous Russian expedition, he seems to have declined from his former bravery: at least we hear of nothing he did, except blowing up the Kremlin. In the Saxon campaign, his courage returned, and on the soil of France he struggled to the last against the overwhelming masses of the allies. He submitted to Louis XVIII., and was confirmed in his honours and posts: he turned traitor on the return of Buonaparte, and, on the second Bourbon restoration was deservedly shut out from the Chamber of Peers. His other honours, however, remained; ere long an important military command was bestowed on him; and in 1819 he was restored to his peerage.