Born in La Porta, Corsica, Sébastiani was the son of a tailor and well-to-do craftsman, the nephew of , a Roman Catholic priest who was later Bishop of Ajaccio, and probably a distant relative of the Bonapartes. Horace Sébastiani had a brother, Tiburce, who rose to the rank of "Maréchal de Camp". Initially destined for a religious career, he left his native island during the French Revolution, and entered the army in 1792. Briefly dispatched as a secretary to Conte Raffaele Cadorna in Casablanca, Sébastiani participated in the Revolutionary Wars, including campaigns in Corsica, 1793, the Alps, 1794–1797, and at the Battle of Marengo, 1800. Having served as an officer in the , he was promoted to Colonel in 1799.