Kenneth Williams was born in 1926 in Bingfield Street, King's Cross, London. The son of barber Charles Williams, he was educated at Lyulph Stanley School. His relationship with his parents, he adored his vivacious mother, Louisa ("Lou"), but hated his morose and selfish father, was key to his personality.

Williams became an apprentice draughtsman to a mapmaker and joined the army aged 18. He was part of the Royal Engineers survey section in Bombay when he first performed on stage, with Combined Services Entertainment along with Stanley Baxter and Peter Nichols.

After the war, his career began with roles in repertory theatre, but few serious parts suited his delivery. His failure to become a serious dramatic actor disappointed him, but potential as a comic performer gave him his break. He was spotted playing the Dauphin in George Bernard Shaw's St Joan in 1954 by the radio producer Dennis Main Wilson, who was casting Hancock's Half Hour.

He lent his distinctive vocal and comedic talents to the series until almost the end of its run, five years later. His nasal, whiny, camp-cockney inflections (epitomised in his "Stop messing about..." catchphrase) became hugely popular with the listening public and would endure in popular lore for many years.

When Hancock decided to move the show away from what he considered to be 'gimmicks' and silly voices, Williams found himself having less to do on the programme. Tiring of his increasingly reduced appearances, Williams joined Kenneth Horne in Beyond Our Ken (1958–1964), and its sequel, Round the Horne (1965–1968).

In the latter, his roles included Rambling Syd Rumpo, the eccentric folk singer; The Amazing Proudbasket, human cannonball; J. Peasemold Gruntfuttock, professional telephone heavy breather and dirty old man; and Sandy of the camp couple, Julian and Sandy (Julian was played by Hugh Paddick), notable for their double entendres and gay slang known as Polari.

Williams appeared in West End revues including Share My Lettuce with Maggie Smith and written by Bamber Gascoigne, and Pieces of Eight, which included material from Peter Cook who was still a student at Cambridge University. The revue included Cook sketches such as One Leg Too Few that would become classics and also starred Fenella Fielding.

Williams' last revue was One over the Eight, with Sheila Hancock. Williams later starred opposite Jennie Linden in My Fat Friend in 1972. He also appeared with Ingrid Bergman in a stage production of George Bernard Shaw's Captain Brassbound's Conversion in 1971.

Williams worked in television and British films, notably the Carry On series with its British "nudge nudge" double entendre-laced humour, which were highly successful but for which he, along with the rest of the cast, was poorly paid.

In his diaries Williams claims he earned more in a British Gas commercial than the entire Carry On series, although that might only be true if one adds the fee he earned from the highly popular spin-off cartoon series Willo the Wisp (taken up by the BBC rather than the commercial TV network).

Despite making a good living, he lived in small flats in north London, the best known location being the now demolished block on Osnaburgh Street.