Im Alter von 30 Jahren beschloss McMillan eine Schauspielerkarriere zu starten. Er besuchte die Schauspielschule Fiorello H. LaGuardia High School of Music & Art and Performing Arts und erhielt Unterricht von Uta Hagen und Irene Dailey. Erst im Alter von 41 Jahren gab er sein Filmdebüt in Serpico.

Kenneth McMillan (July 2, 1932 – January 8, 1989) was an American actor. McMillan was usually cast as gruff, hostile and unfriendly characters due to his rough image. However, he was sometimes cast in some lighter comic roles that highlighted his gentler side. He was perhaps best known as Jack Doyle in Rhoda (1977–1978), and as Baron Harkonnen in David Lynch's Dune.