He also played Victor Meldrew's long-suffering neighbour Patrick Trench in the comedy series One Foot in the Grave, and George Windsor in the final three series of Waterloo Road.
The youngest of three sons of a Prudential plc insurance broker/manager and a home economics school teacher, Deayton was brought up in Banstead, Surrey, and attended Oakhyrst Grange School and Caterham School. He showed early promise as a footballer, and had a trial with Crystal Palace. He was captain of the Caterham U16 Rugby team.
Deayton read languages at New College, Oxford, where he was recruited into the Oxford Revue, performing with them at the Edinburgh Festival Fringe. This led to the creation of the parody band the Hee Bee Gee Bees in 1980, with the songs written by Richard Curtis and Philip Pope. Their best-selling single "Meaningless Songs (In Very High Voices)" (plus the B-side "Posing in the Moonlight") was a parody of the falsetto style of disco hits by the Bee Gees.
Deayton founded his career on Radio Active, a parody of British local radio stations broadcast on BBC Radio 4 between 1981 and 1987, which he co-wrote and performed. It transferred to television as KYTV between 1989 and 1993. Deayton presented a tribute to Radio Active and KYTV colleague and friend (and long-time BBC producer) Geoffrey Perkins for BBC Radio 4 on 4 October 2008.
Deayton was frequently a straight man alongside Rowan Atkinson. He starred with Atkinson as a pool attendant and a man on a park bench in the Mr. Bean episode "The Curse of Mr. Bean" and appeared opposite Atkinson in the Black Adder episode "Born to be King" (1983) as one of the Jumping Jews of Jerusalem.
From 1988–91, Deayton was a featured player in all three series of the Emmy award-winning sketch comedy programme Alexei Sayle's Stuff.
In 1990, Deayton was cast as the Meldrews' neighbour Patrick Trench in the British suburban sitcom One Foot in the Grave and was selected as host of Have I Got News for You. The same year, he featured on television advertising the Vauxhall Nova. Andre Ptaszynski tried to persuade him to take the lead role in Steven Moffat's sitcom Chalk, a role eventually taken by David Bamber.
Deayton's suave manner as host of Have I Got News for You led to his being nicknamed "TV's Mr Sex", by a Time Out listings writer. He was much in demand as a presenter of television specials including the BBC's New Year's Eve show and the BAFTA Awards. He also featured in a series of advertisements for Barclaycard and the films Savage Hearts and Elizabeth.
In an episode of Coupling, he appears in a fantasy sequence with Mariella Frostrup. In addition, he hosted the late-1990s BBC show Before They Were Famous, which showed early and frequently embarrassing clips of TV and film stars (including Deayton himself) when they were relatively unknown.
In May 2002, the News of the World suggested he had taken cocaine and had sex with prostitutes. He was ridiculed by Paul Merton and Ian Hislop in the following episode of Have I Got News For You but continued as presenter. Deayton began the episode with: "Good evening and welcome to Have I Got News For You, where this week's loser is presenting it." He added later, "For those watching at home, don't adjust your sets, my face really is this red." After more allegations in October, Deayton was sacked two episodes into the new series after Merton and Hislop repeatedly implied during the programme that Deayton should resign.
After Have I Got News for You, his work included a reunion of the Radio Active cast in a new episode in December 2002. In 2003, he guest-starred as Downing Street's spin doctor in an episode of the BBC comedy Absolute Power, starring Stephen Fry and John Bird. In January 2004, he starred in the BBC comedy Nighty Night. Deayton had a cameo role as a hotel receptionist in the 2004 Fat Slags film. A few months later, he presented the quiz Bognor or Bust. In January 2006, he hosted an ITV show based upon self-help videos called Help Your Self.
Deayton is associated with Comic Relief/Sport Relief and featured in its broadcasts. He co-presented the Sport Relief charity programme Only Fools on Horses in July 2006. Deayton appeared for the England team as a second-half substitute in the Soccer Aid match in support of UNICEF on 27 May 2006. He returned as a starting player for England in the 7 September 2008 rematch.
In 2007, he was in Casualty, playing an exaggerated version of himself in a Comic Relief-related story. In June 2007, Deayton returned to the BBC to host panel show, Would I Lie to You?. In November 2007, he was censured by the BBC for making a "pungently personal" joke about Jimmy Savile and his mother on the show. Deayton left the show in 2009 and was replaced by Rob Brydon. On 3 September 2007, Deayton hosted the third series of Hell's Kitchen, but was sacked in 2009 after arguments with chef Marco Pierre White and was replaced by Claudia Winkleman.
In 2008, Deayton also presented Comedy Sketchbook, a nostalgic look at classic comedy sketches, on BBC1. On 6 December 2008, he presented the 2008 British Comedy Awards, after host Jonathan Ross stepped down because of controversy surrounding The Russell Brand Show prank telephone calls row.
His feature film appearances include the mysterious, all-knowing man in That Deadwood Feeling (2009, co-starring Jack Davenport, Dexter Fletcher and David Soul), Swinging with the Finkels (2011, written and directed by Jonathan Newman, with Mandy Moore and Martin Freeman), and Playing the Moldovans at Tennis (2012). He returned to BBC Radio 4 in 2011 to host the panel show It's Your Round.
On 12 December 2012, Deayton joined the BBC drama series Waterloo Road as a cynical teacher and for a further season as deputy head, George Windsor. He made his first appearance in episode 27 of the eighth series in 2013. In December 2012, he appeared on the BBC Two series World's Most Dangerous Roads, in which he and Mariella Frostrup were filmed driving along the east coast of Madagascar. He co-stars with Anna Chancellor in the BBC sitcom Pramface (2013).
In August 2016, at the Edinburgh Festival, Deayton wrote and performed in a revival of Radio Active.
At Oxford, Deayton was in a relationship with actress Helen Atkinson-Wood (a co-star on Radio Active and KYTV). While touring with the HeeBeeGeeBees in Australia in the 1980s, Deayton saved Atkinson-Wood's life when he rescued her after she was caught in a rip tide while swimming off Sydney’s Manly Beach. Deayton later lived with 1970s singer/songwriter Stephanie de Sykes.
From 1991 to 2015, he was in a long-term relationship with scriptwriter Lise Mayer.
Deayton is a Manchester United supporter.
In May 2002, the British tabloid newspaper News of the World published allegations about Deayton's affair with a woman who later claimed she was a prostitute and about his use of cocaine. In October there were further reports of another affair, and he was asked to step down as the quizmaster of Have I Got News For You on 29 October 2002. Deayton was dismissed. One online poll, on the BBC's own website, showed over three-quarters of respondents wanting Deayton to stay on as the programme's host.
In April 2003, Stephen Fry supported Deayton, by refusing to appear on the show again.