Fox's made his theatrical début in 1958The Loneliness of the Long Distance Runner (1962). Throughout the 1960s he worked mostly on stage, including a turn as Hamlet. In the late 1960s and early 1970s he established himself with roles in major British films including Oh! What a Lovely War (1969), Battle of Britain (1969) and The Go-Between (1970).
, and his first film appearance was as an extra in
From then onwards, he was much sought after, appearing in such films as A Bridge Too Far (1977) as Lieutenant General Horrocks — a role he has cited as a personal favorite. and for which he won the Best Supporting Actor award at the British Academy Film Awards. He also starred in Force 10 From Navarone (1978), with Robert Shaw and Harrison Ford.
He portrayed King Edward VIII in the television drama, Edward and Mrs. Simpson (1978). In the film Gandhi (1982), Fox portrayed Brigadier-General Reginald Dyer, responsible for the Amritsar Massacre in India. He then appeared as M in the unofficial James Bond film Never Say Never Again (1983), a remake of Thunderball (1965). He also appeared in The Bounty (1984), with Laurence Olivier, Wild Geese II (1985) and in The Importance of Being Earnest (2002), Nicholas Nickleby (2002), and Stage Beauty (2004).