McKeown co-wrote the screenplay for Brazil (1985) with Gilliam and Tom Stoppard, for which they were collectively nominated for an Academy Award. In The Adventures of Baron Munchausen (1988) McKeown again co-wrote the screenplay with Gilliam. After nearly twenty years apart, the two collaborated on the screenplay for The Imaginarium of Doctor Parnassus.

Other notable screenwriting credits for McKeown include Plunkett & Macleane (1999) and Ripley's Game (2002). He went uncredited for his work on Batman.

McKeown has had a number of acting roles in films and television series associated with the Monty Python comedy group. He started off his career with minor roles on Ripping Yarns (1977), which starred Michael Palin and Terry Jones, and Fawlty Towers (1979), which starred John Cleese. Around the same time, he played some minor roles in Life of Brian, where he met Gilliam.

Since then, he has had acting roles in a number of the films he helped to write with Gilliam, including an appearance as Harvey Lime in Brazil and as Adolphus and Rupert in The Adventures of Baron Munchausen. In addition, he's had roles in other Python-associated films such as Time Bandits (1981), The Missionary (1982), A Private Function, Erik the Viking (1989), and American Friends (1991).

He's also had some film roles not associated with the Monty Python group such as his role as Jerry Hadley in Spies Like Us (1985) and Mr. Cunliffe in Prick Up Your Ears (1987).

McKeown has also appeared on numerous television series including: Yes Minister (1980), The Hitch Hiker's Guide To The Galaxy (1981), Pinkerton's Progress (1983 which he also wrote for) and The Young Indiana Jones Chronicles (1992).