Born in Paris on 8 December 1761, Antoine-Louis Decrest de Saint-Germain descended from a noble family of the Ancien Régime.

In April 1794, he was arrested due to his aristocratic ascendance and was reintegrated in the army only in August 1795, but, in exchange, was immediately promoted to the rank of colonel and given the command of a chasseurs à cheval regiment.

In this capacity, Saint-Germain fought in campaigns on the Rhine and received a first battle wound when a cannonball broke his right leg on 20 September 1796.

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 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.