Born to a large Irish Catholic family (the fifth of nine children) and raised in New York City, Lydon overcame a birth defect and alcoholic father to begin a Broadway career in 1937, acting opposite Van Heflin, Sidney Lumet and Uta Hagen in separate productions. After a number of films with Paramount and RKO, Lydon hit his stride in the "Henry Aldrich" B movie series of the early 1940s. After working increasingly in television in the early 1950s, Lydon turned to production roles, helping to create M*A*S*H (1972) and 77 Sunset Strip (1958). He is still active as a producer and writer.