Born in La Porta, Corsica, in 1771, Sébastiani was the son of a tailor and well-to-do craftsman, the nephew of Louis Sébastiani de La Porta, 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.
In 1789 he entered the French army. In 1793, as a French lieutenant, he took part in the war in his native island, after which he served in the Army of the Alps. He became chef de brigade in 1799. Attached by birth and service to the future Emperor Napoleon, he took part in the Coup d'Elat of 18th Brumaire (9th November 1799). He was present at Marengo in 1800. Sebastiani next appears in his first diplomatic post, in Turkey and Egypt (1802). Promoted general of brigade in 1803, he served in 1805 in the first of the great campaigns of the Empire. His conduct at Austerlitz (2nd December), where he was wounded, won him promotion to the rank of general of division. Sebastiani soon returned to Constantinople as French Ambassador. As ambassador he induced the Porte to declare war on Russia, as a soldier he directed with success the defence of Constantinople against the British squadron of Admiral (Sir) J. T. Duckworth. But the deposition of the Sultan Selim III put an end to French diplomatic success in this quarter, and Sebastiani was recalled in April 1807 (see La Politique orientate de NapoUon: Sebastiani et Gardam, by E. Driault, Paris, 1905). He was at this time made Count of the Empire. As the commander of a corps he served in the Peninsular War, but his cavalry genius did not shine in thelaborious and painful operations against the careful English and the ubiquitous guerrilleros. In the more congenial grande guerre of Russia and Germany he was in his element, and at Smolensk, Borodino and Leipzig he did brilliant service. He accepted the Restoration government in 1814, but rejoined his old leader on his return from Elba. After Waterloo he retired into England for a time, but soon returned, and was placed on half-pay. From 1819 onwards he was a prominent member of the Chamber of Deputies. He held the posts of Minister of Marine, and, later, of Foreign Affairs. In this latter capacity he was the author of the historic saying "Order reigns at Warsaw. " In 1832 he was a Minister of State without portfolio, next year ambassador at Naples, and from 1833 to 1840 was ambassador to Great Britain. On his retirement from this post he was made Marshal of France. He was a brilliant social figure in Paris. His last years were clouded by the death of his daughter at the hands of her husband, the due de Praslin. He died at Paris on the 21st of July 1851. His brother, Jean Andre Tiburce Sebastiani (1786 - 1871), entered the army in 1806, served in the Peninsula from 1809 to 1811, and in the great campaigns of Russia, Germany, France and Belgium. He took part in the war of Greek independence under General Maison. In 1842, now lieutenant-general and peer of France, he was appointed to command the military division of Paris. But he proved incapable of dealing with the Revolution of 1848, and the remainder of his life was spent in retirement in Corsica.
Horace Sébastiani's name is inscribed on the western side of the Arc de Triomphe. An avenue in Bastia was named in his honor (Avenue Maréchal Sébastiani).
In 1938, Rachel Field published her All This and Heaven Too, a novel which centers on the killing of Sébastiani's daughter. The 1940 drama film of the same starred Barbara O'Neil as Fanny, Charles Boyer as De Choiseul-Praslin, and Bette Davis as Henriette Deluzy-Desportes, the governess. Sébastiani is portrayed by Montagu Love.
Sébastiani is also one of the protagonists in Prince Michael of Greece's novel Sultana - La Nuit du Sérail, which depicts fictionalized events of Selim III's rule. In 1989, the story was later turned into an American-Swiss film co-production: titled The Favorite (or La Nuit du Sérail), it starred Laurent Le Doyen as Sébastiani.
Starting in 1819, after being promoted by the Duke Decazes, he was a prominent member of the Chamber of Deputies, initially representing Corsica, rallying with the Left. According to the Revue's comments, his political choice was unusual, reportedly astonishing both members of the Left and the moderate Decazes, a Royalist. Inside the Chamber, he joined forces with Maximilien Sebastien Foy, notably pushing projects to recognize the merits of Grande Armée veterans; a speech he held on the latter occasion, which gave praise to the French tricolor, caused an uproar among conservative deputies.
In 1806, Sébastiani married Jeanne-Françoise-Antoinette (Fanny) Franquetot de Coigny, only daughter of François-Henri de Franquetot, marquis de Coigny). She died in childbirth while in Constantinople, just a few days before the Sultan was deposed (14 April 1807), and left Sébastiani a large fortune. Upon hearing news of her death, Sultan Selim transmitted condolences through his Dragoman.
