Born in Paris as the son of Arthur Lelong, the owner of a textile shop, he trained at the Hautes Etudes de Commerciales in Paris and opened his fashion house in the early 1910s. The first Lelong designs were featured in Vogue magazine in 1913.  Poor health caused the end of his career; Lelong retired in 1952.
Lelong did not actually create the garments that bore his label. "He did not design himself, but worked through his designers," wrote Christian Dior, who was a member of the Lelong team from 1941 until 1946, during which time he created the collections in collaboration with Pierre Balmain. "Nevertheless," Dior continued, "in the course of his career as couturier his collections retained a style which was really his own and greatly resembled him." Other designers who worked for Lelong included Nadine Robinson and Hubert de Givenchy.
Lelong was married three times.
His first wife, whom he divorced on 16 July 1927, was Anne-Marie Audoy.
He married on 10 August 1927, as his second wife, Princess Natalie Paley (1905–1981), who had worked as a saleswoman in the Lelong perfume department. She was a daughter of Grand Duke Paul Alexandrovich of Russia and his morganatic wife, Olga Karnovich. They divorced in 1937.
Lelong died of a heart attack in Anglet, France.