In addition to Gunsmoke, the show that truly made him a legend (he received an Emmy Award for Outstanding Performance by an Actor in a Supporting Role in a Drama in 1968), Milburn had a lot of acting gigs on his resume between the 1930s, 1940s, and 1950s.
Some of those roles include Sky Patrol, Stunt Pilot, The Three Mesquiteers, A Doctor’s Diary, Atlantic Flight, Climax!, Drango, TV Reader’s Digest, The Man Behind the Badge, Fireside Theatre, Adventures of Wild Bill Hickok, and many more.
Source : https://www.distractify.com/p/why-did-doc-leave-gunsmoke