Despite the show’s success, and perhaps due to its long run, M*A*S*H experienced several significant cast changes, and several favorite characters were replaced with equally dynamic new ones — a standard practice on long-running shows today, but rare back then. In addition to Stevenson’s departure, Wayne Rogers (Captain “Trapper” John McIntyre) abruptly quit after season three, reportedly due to dissatisfaction with the development of his character. (Production planned a multimillion-dollar breach-of-contract lawsuit against the actor, but he revealed he never signed his contract, which made the matter moot.)
Stevenson’s Colonel Blake was succeeded by Colonel Potter (played by Harry Morgan), while B.J. Hunnicutt (played by Mike Farrell) took Trapper John’s place in the 4077th. Radar, the only main character transferred from the movie to the TV series, left the show in 1979.
Of the many actors who appeared on the show, Alda was the only star to appear in every episode. Through its run, the actor took increasing creative control of the series, directing 31 episodes including the finale, and co-writing 13 episodes. He became the first person ever to win Emmy Awards for acting, directing, and writing for the same show.
Loretta Swift (Margaret “Hot Lips” Houlihan) was a close second in terms of longevity; she appeared in all 11 seasons but missed a handful of episodes along the way.