Vandermaesen joined the Army in 1782 in the Touraine Regiment as a foot soldier, rising to lieutenant in 1792. He was employed as an instructor for the Jura battalion, then put in charge of the logistics, and eventually took command of the Jura battalion after a promotion to adjudant-major. He took part in the Battle of Wissembourg and in engagements in Haguenau, Vandenheim and Landau. General Michaud gave him command of the grenadiers of the division and sent him to capture the town of Brumpt, where a violent engagement occurred; Vandermaesen was wounded by a gunshot in the leg.