Born in London, he was the son of theatrical agent John Cadell, grandson of the Scottish character actress Jean Cadell, the brother of the actress Selina Cadell and commercials director Patrick Cadell, the cousin of the actor Guy Siner and son-in-law of the television producer David Croft. He was educated at The Hall School in Hampstead and Bedales School at Petersfield where his close friends included 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 the animated adaptation of Watership Down (1978), based on the novel by Richard Adams. Subsequently he had roles in television programmes such as Enemy at the Door (1978–80), and also appeared briefly in the disaster film Meteor (1979) as a TV news reporter. He is best remembered for his role as the well-meaning holiday camp manager Jeffrey Fairbrother in the BBC situation comedy Hi-de-Hi! (1980–84) and for playing the disingenuous civil servant Dundridge in the screen adaptation of a novel by Tom Sharpe, Blott on the Landscape (1985). On radio he played the elven-king Celeborn in the BBC adaptation of The Lord of the Rings (1981). He starred in the BBC sitcom Life Without George (1987-1989) which ran for three series and also featured Carol Royle in a leading role.

Cadell appeared in the 1991 British comedy series Singles. Other television credits include, 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. He also narrated the children’s television series Bump for the BBC which is all about a baby elephant (who always bumps into things) and his friend Birdie. In 1985 Cadell married actress Rebecca Croft, the daughter of Dad’s Army co-creator David Croft. The couple had two sons. In January 1993 Cadell suffered a near-fatal heart attack after giving a recital with Joanna Lumley at the Queen Elizabeth Hall, in London. The doctors singled out smoking as a contributory cause – at one stage Cadell smoked 80 cigarettes a day – but he was back at work playing in Travels with My Aunt (for which he won the Laurence Olivier Award for Best Comedy Performance) at the Wyndhams Theatre four months after a triple bypass operation. On 11 September the same year, cancer was detected. On 6 March 1996 Cadell died at the age of 45, 12 weeks before his best friend Jeremy Sinden also died of lung cancer. Sinden had been born 5 weeks before him, and his father, Donald Sinden, had been a client of Cadell’s father John, for over 30 years.