The son of an advocate, Joubert was born at Pont-de-Vaux (Ain), and ran away from school in 1784 to enlist in the artillery. He was brought back and sent to study law at Lyon and Dijon. In 1791, during the French Revolutionary Wars, he joined the French Revolutionary Army regiment of the Ain, and was elected by his comrades successively corporal and sergeant. In January 1792 he became sous-lieutenant, and in November lieutenant, having in the meantime experienced his first campaign with the army of Italy.