Born in London, he was the grandson of the Scottish character actor Jean Cadell and the brother of the actress Selina Cadell. He was educated at Bedales School at Petersfield where his close friends included the writer and broadcaster (and one time Member of the British Parliament) Gyles Brandreth, who remained a friend until Cadell's death. Cadell trained at the Bristol Old Vic Theatre School. His first successes were found in the theatre in the mid to late 1970s. He then provided the voice of Blackberry in Watership Down, the animated adaptation of Richard Adams' novel of the same name, and went on to play parts in a number of British television programmes, including series such as Enemy at the Door. He also appeared in a few films. He is most remembered for his role as the well-meaning but out-of-place holiday camp manager Jeffrey Fairbrother in the BBC Situation comedy Hi-de-Hi!, and for playing the disingenuous civil servant Dundridge in the screen adaptation of Blott on the Landscape, both in the mid-1980s. On radio he played the elven-king Celeborn in the 1981 BBC adaptation of The Lord of the Rings. He starred in the BBC sitcom Life Without George alongside Carol Royle which ran for two series. He appeared in the British comedy series Singles and cameoed in several shows, including Minder, Bergerac, The Kenny Everett Television Show, and Roald Dahl's Tales of the Unexpected. He was also in heavy demand as a voice-over for television commercials.