The actor played a borough commander in The Taking of Pelham One Two Three, but often was cast as characters such as a cowardly small town sheriff in Tobe Hooper's 1979 TV mini-series Salem's Lot, a similar law enforcement officer in the 1987 Burt Reynolds film Malone, William Hurt's bitter paraplegic father in Eyewitness, a wily safe cracker in The Pope of Greenwich Village, and a racist fire chief in Ragtime who is memorably told off by the New York City police commissioner, played by James Cagney.

In 1985, he portrayed New York City's newly appointed police commissioner in the short-lived television crime drama Our Family Honor.

He portrayed the grotesquely obese and gleefully psychotic Baron Vladimir Harkonnen in Dune, the pathetic drunken pop of Aidan Quinn in Reckless and as Cressner, a sleazy high roller gambler in "The Ledge," a segment of the horror anthology film Cat's Eye.

McMillan was also adept at comedy, giving performances as a baseball club manager in Blue Skies Again, Meg Ryan's corrupt security guard captain dad in Armed and Dangerous and a dotty senile veterinarian in Three Fugitives.

Among the TV shows McMillan did guest spots on are Dark Shadows, Ryan's Hope, as a 53rd precinct lieutenant on Kojak, Starsky & Hutch, The Rockford Files, Moonlighting, Lou Grant, Magnum, P.I.