Born in Paris, son of Henri François, comte de Ségur, and his wife Philippe Angélique de Froissy, he was appointed to the command of an infantry regiment at eighteen, and served under his father in Italy and Bohemia. He was wounded at Roucoux in Flanders in October 1746, and lost an arm at Lauffeld in 1747. In 1748 he succeeded his father as lieutenant-general of Champagne and Brie; he also received in 1753 the governorship of the county of Foix.