After studying at the Sorèze Military College (1793–1798), he joined the 1st Hussards Regiment as a volunteer on 3 September 1799. He served under General Jean-Mathieu Seras, who promoted him to the rank of sergeant on 1 December 1799. In the same month, on 31 December 1799, he was promoted to the rank of second lieutenant in recognition of his courage. He fought with the Army of Italy and took part in the Battle of Marengo and the Siege of Genoa, during which his father, General Jean-Antoine Marbot died.