Rosten was an inveterate Anglophile. He had enjoyed his years at LSE, was amazed by the enthusiastic reception Kaplan had received in the English press, and returned to London whenever opportunity dictated and even when it didn't. He lived in considerable luxury in a penthouse flat in Sutton Place, one of the most exclusive areas of New York, and rented a mews flat in Mayfair. England represented the tranquillity he could not find in America. He loved to rummage in English bookshops and wear English clothes - he contrived to display a subdued elegance - to go to the London theatres and entertain and be entertained in London clubs. He himself was a member of the Savile, the Reform and the Garrick.
Rosten was born into a Yiddish-speaking family in Łódź, Russian Empire (now in Poland), but emigrated to the United States with his family in 1911 when he was three. His parents were Samuel C. Rosenberg and Ida (Freundlich) Rosenberg, both trade unionists. They opened a knitting shop in the Greater Lawndale area of Chicago, where Rosten and his younger sister grew up among other working-class Jewish families.
Maxim Lieber served as his literary Editor, 1935-1938.
Rosten was a successful Screenwriter. He wrote the story for The Dark Corner (1946), a film noir starring Mark Stevens; and Lured, the Douglas Sirk-directed period drama starring Charles Coburn; both films featured Lucille Ball. He is listed as one of the Writers for Captain Newman, M.D. (1963) adapted from his novel of the same title. Other films: Mechanized Patrolling (1943) (as Leonard Q. Ross), They Got Me Covered (1943) (story) (as Leonard Q. Ross), All Through the Night (1942) (story) (as Leonard Q. Ross), The Conspirators (1944) (screenplay), The Velvet Touch (1948), Sleep, My Love (1948) (novel) (screenplay), Double Dynamite (1954) (story), Walk East on Beacon (1952), and Mister Cory (1957) (story).
He is also well known for his encyclopedic The Joys of Yiddish (1968), a guide to Yiddish and to Jewish culture including anecdotes and Jewish humor. It was followed by O K*A*P*L*A*N! My K*A*P*L*A*N! (1976), a reworking of the two 1930s collections, and Hooray for Yiddish! (1982), a humorous lexicon of the American language as influenced by Jewish culture. Another Rosten work is Leo Rosten's Treasury of Jewish Quotations.
Rosten died in New York City in 1997. His obituary in The Independent on February 21, 1997, written by Chaim Bermant, describes his personality as follows: