He was born in London to theatrical agent Robin Fox and actress Angela Worthington. He is a brother of actor Edward Fox and film producer Robert Fox. He is also a paternal half-brother of Daniel Chatto and a brother-in-law of Lady Sarah Chatto. The actress Emilia Fox is his niece.

His early film appearances were made under the name William Fox. During the 1960s he gained popularity and appeared to be heading for film stardom. His appearances in films such as Thoroughly Modern Millie (1967) and Performance (1970) (alongside Mick Jagger), as well as his relationship with actress Sarah Miles, had made him a media personality. It was then that he decided to give up acting altogether to devote himself to charitable work.

After an absence of about ten years, Fox gradually returned to the screen, appearing with his brother Edward in A Passage to India (1984) and playing Anthony Blunt in the acclaimed BBC play by Alan Bennett, A Question of Attribution (1992). More recently, he has appeared in Charlie and the Chocolate Factory (2005) playing Mr. Salt, Veruca Salt's father.

He appeared in the Doctor Who audio drama Shada.