Burly, crude-featured character actor McMillan graduated from the celebrated High School for the Performing Arts in New York. He spent years holding down odd jobs while trying unsuccessfully to find work as an actor. In 1960 he quit his job and joined a touring production of Sweet Bird of Youth. This led to work in off-Broadway shows and TV soap operas. He finally debuted on Broadway in Borstal Boy (1970), then debuted onscreen in a minor role in Serpico (1971). In the late '70s he worked with the New York Shakespeare Festival, and was in the cast of the TV sitcom Rhoda. From 1978-85 he was a busy screen actor. The first major attention he got onscreen came with his portrayal of a bigoted, beer-bellied fire chief in Ragtime (1981). He was the father of actress Allison McMillan.