Tony Slattery, who has died of a heart attack aged 65, showed his great talent for improvisational comedy on the Channel 4 show Whose Line Is It Anyway? He often appeared as one of the four performers creating characters, scenes and songs based on suggestions by its host, Clive Anderson, or the studio audience.

“Whose Line is just four people and a couple of stools,” Slattery enthused. “It’s just a brilliantly simple idea. The audience love to see you thinking on your feet.”

Those audiences warmed to Slattery’s outrageous patter. Anderson recalled: “He would not use two words if a ruder one would do – the naughtiest kid in the class, adored by the studio audience and those watching on TV.”

The game show also featured John Sessions, Paul Merton and Josie Lawrence, and had previously run on BBC radio without Slattery. He joined the first TV series in 1988 after a stint on the London comedy circuit with the pianist Richard Vranch. He identified the programme as the perfect showcase for his skills, sent a video CV to the producers, and was signed up. When he was dropped after the seventh series, in 1995, viewing figures fell.

During his time on the show, he seemed omnipresent on British television – as sitcom actor, film critic, gameshow host and chatshow guest – leading one journalist to describe him as “a light entertainment producer’s wet dream, a telly tart who can’t say no”.

But this suddenly ended when he had a nervous breakdown in 1996, having coped with his punishing schedule by becoming hooked on cocaine – sometimes spending £4,000 a week – and drinking two bottles of vodka a day, addictions that left him bankrupt.

He spent six months as a recluse – “in a pool of despair and mania”, he said in a 2006 BBC documentary, The Secret Life of the Manic Depressive. Eventually, a friend persuaded him to seek psychiatric help. He quit the drugs, but not the alcohol, and was diagnosed with bipolar disorder. “The black dog got me in its jaws and just hung on for two years,” he told the Independent in 1998.

His attempts at a comeback never brought him the same level of celebrity. “I had a very happy time until I went slightly barmy,” he told the Guardian in 2019, when he joined some of his former Whose Line colleagues for a reunion at the Edinburgh festival. In the same interview, the star revealed that he had been abused by a priest at the age of eight, but never told his parents. Following this revelation, he was the subject of a BBC2 Horizon documentary, What’s the Matter with Tony Slattery? (2020), which explored his psychological problems and childhood trauma.

Born in north London, he was the youngest of four boys and one girl of Northern Irish parents, Margaret (nee O’Malley), a home help, and Michael Slattery, who worked in a Heinz factory. He attended Gunnersbury grammar school and excelled at judo, becoming an under-15 champion and reaching black belt status at 17.

Winning an exhibition to study modern and medieval languages at Trinity Hall, Cambridge, he soon found himself taking part in the university’s revue group. He recalled: “I bumped into Stephen Fry in the street … ‘Dear boy,’ he said, ‘you must come and audition for the Footlights.’” Other contemporaries included Hugh Laurie, Emma Thompson, Jan Ravens, Sandi Toksvig and Vranch.

Slattery made an immediate impact in his first show. “He tore up the audience with guitar songs and extraordinary monologues of his own devising,” wrote Fry in his autobiography, The Fry Chronicles.

When Fry, Laurie, Thompson and Slattery, with Vranch directing a five-piece band, were among the Footlights cast performing a revue, The Cellar Tapes, at the Edinburgh fringe in 1981, they won the first Perrier award. Slattery was made president of the Footlights society the following year.

In London, after abandoning Aftertaste, his double act with Vranch, Slattery appeared in Noel Gay’s stage musical Me and My Girl (Adelphi theatre, 1986), displaying a fine baritone voice. He starred in another Gay musical, Radio Times (Queen’s theatre, 1992), and two years later earned an Olivier nomination in Tim Firth’s play Neville’s Island (Apollo theatre, 1994).

On television, during his Whose Line Is it Anyway? fame, Slattery took over as Fry’s spoof investigative reporter in This Is David Lander (Channel 4), with a title change to This Is David Harper (1990).

He also had a presenting role – sometimes with Vranch – in the ITV quiz The Music Game (1992-93), and hosted the BBC etiquette show Ps and Qs (1992).

The 1992 film Peter’s Friends, about old college chums meeting up after 10 years, saw Slattery himself reunited with his Cambridge friends Fry, Laurie and Thompson, significantly cast as a social outsider. He said of his alumni: “I wasn’t part of their class.”

Post-breakdown, Slattery’s return to TV saw him taking dramatic rather than comedy roles. Among other shows he popped up in was Casualty (2003), and he played Eric Telford, a bookie dating Carol Baldwin, in Coronation Street (2005-06). He was also in the 2005 television film Ahead of the Class, starring Julie Walters.

He then appeared as Sidney Snell, a frequent – and bad-smelling – client of Fry’s Norfolk solicitor, in all three series of Kingdom (2007-09), but it proved to be his last screen role. A brief return to the stage came in 2017 with his comedy show Slattery Night Fever at the London Improv theatre. In 2024 he launched a podcast, Tony Slattery’s Rambling Club.

He is survived by Mark Michael Hutchinson, an actor and his partner since 1986.

• Tony Slattery, actor and comedian, born 9 November 1959; died 14 January 2025