Trevor was born Claire Wemlinger. After attending Columbia and the American Academy of Dramatic Arts, she began her acting career in the late '20s in stock. By 1932 she was starring on Broadway; that same year she began appearing in Brooklyn-filmed Vitaphone shorts. She debuted onscreen in feature films in 1933 and soon became typecast as a gang moll, a saloon girl, or some other kind of hard-boiled, but warm-hearted floozy. Primarily in B movies, her performances in major productions showed her to be a skilled screen actress; nominated for Oscars three times, she won the Best Supporting Actress Oscar for her work in Key Largo (1948). In the '50s she began to appear often on TV; in 1956 she won an Emmy for her performance in Dodsworth opposite Fredric March.