Duncan worked as physical director at the McFadden Physical Culture Health Home, and wrote for a magazine about physical culture. He also operated a gymnasium in Philadelphia and was a professional wrestler. Duncan debuted in
vaudeville in 1906 as part of a strongman act. He began to act in silent films, and had roles in hundreds of serial episodes, many of them Westerns. His career spanned the change to "talkies", and he acted from 1911 to 1940. When William Duncan joined
Vitagraph, his contract was worth $1,000,000 a year. He was better paid than
Mary Pickford and
Douglas Fairbanks. He appeared in films with Olympic swimmer
Buster Crabbe (who played
Flash Gordon in the 1930s). He was also a prolific writer and director of films. Duncan followed the film industry to California and lived and worked there for most of his life. At his death, he was interred at
Inglewood Park Cemetery,
Inglewood, California, USA. ==Family==