In April 1805, Saint-Germain was named general of brigade and given a command in Etienne de Nansouty's 1st heavy cavalry division and thus campaigned with the Grande Armée during the War of the Third Coalition and War of the Fourth Coalition during 1805-1806. A Baron of the Empire in January 1809, Saint-Germain led a brigade of Nansouty's cuirassiers at the battle of Aspern-Essling and was promoted to general of division in July 1809. In 1812, he took part to the Campaign in Russia, receiving a wound at the battle of Borodino. In September 1813, Saint-Germain was created a Count of the Empire and took part to the battle of Hanau, where he had a hand in forcing the passage of the debris of the Grande Armée towards France. During the 1814 campaign in France, he led several noted cavalry actions. During the Bourbon Restoration he fared well and in 1818 was named Inspector general of cavalry.