He was born in King's Cross, London, where his father kept a hairdresser's shop. He joined the army aged 18, and first performed on stage with Combined Services Entertainment while stationed in Bombay.
His professional acting career started in 1948 in repertory theatre, but from 1954 he was appearing on BBC radio in Hancock's Half Hour. He appeared regularly in Keeneth Horne's Beyond our Ken from 1958 to 1964, and its successor Round the Horne from 1965 to 1968, playing a number of characters with funny voices. He is particularly remembered for the "Julian and Sandy" sketches, in which he played Sandy opposite Hugh Paddick as Julian. The pair were portrayed as a gay couple (homosexuality ws still illegal until 1967) and made much use of Polari.
From 1958 to 1978 he was a regular in the Carry On films, appearing in 26 of them, more than any other actor. He appeared regularly on Just a Minute on BBC Radio 4 from 1968 until his death.
On 14 October 1962, Williams's father, Charlie, was taken to hospital after drinking carbon tetrachloride that had been stored in a cough mixture bottle. Williams refused to visit him, and the following day went out for lunch and then to the cinema. Charlie died that afternoon and, an hour after being informed, Williams went on stage in the West End. The coroner's court recorded a verdict of accidental death due to corrosive poisoning by carbon tetrachloride.
Several years later Williams turned down work with Orson Welles in America because he did not like the country and had no desire to work there. Many years after his death, The Mail on Sunday, quoting Wes Butters, co-writer of the book Kenneth Williams Unseen: The Private Notes, Scripts And Photographs, claimed Williams had been denied a visa because Scotland Yard considered him a suspect in his father's death.
Williams insisted that he was celibate, and his diaries substantiate his claims – at least from his early 40s onwards. He lived alone all his adult life and had few close companions apart from his mother, and no romantic relationships of significance. His diaries contain references to unconsummated or barely consummated homosexual dalliances, which he describes as "traditional matters" or "tradiola" (since male homosexual activity was a criminal offence in the UK before 1967, outright admission would have been held against him if anyone had read the diaries). He befriended gay playwright Joe Orton, who wrote the role of Inspector Truscott in Loot (1966) for him, and had holidays with Orton and his lover, Kenneth Halliwell, in Morocco.
Williams lived in a succession of small rented flats in central London from the mid-1950s. After his father died, his mother, Louisa, lived close by him and, finally, in the flat next to his. His last home was a flat on Osnaburgh Street, now demolished.
Williams rarely revealed details of his private life, though he spoke openly to Owen Spencer-Thomas about his loneliness, despondency and sense of underachievement in two half-hour documentary programmes entitled Carry On Kenneth on BBC Radio London.
In later years his health declined, along with that of his elderly mother, and his depression deepened. He died on 15 April 1988 in his flat - his last words (recorded in his diary) were "Oh, what's the bloody point?" - the cause of death was an overdose of barbiturates. An inquest recorded an open verdict, as it was not possible to establish whether his death was a suicide or an accident. Dr John Elliott, deputy coroner for inner north London said: "The cause of death was a barbiturate overdose. Where Mr Williams would have got these from we would not be able to establish. There is no indication given as to why he should have taken this overdose and therefore I record an open verdict." His diaries reveal he had often had suicidal thoughts and as far back as his earliest diaries he noted there were times when he could not see any point in existence. His authorised biography argues that Williams did not take his own life but died of an accidental overdose. The actor had doubled his dosage of antacid without discussing this with his doctor, which, combined with the aforementioned mixture of medication, is the widely accepted cause of death. He had a stock of painkilling tablets and it is argued that he would have taken more of them if he had been intending suicide. He was cremated at East Finchley Cemetery and his ashes were scattered in the memorial gardens.
- Geoffrey Wansell, "Did Kenneth Kill Himself?" Daily Mail 30 November 2005 "... in October 1962, Charlie Williams died after drinking a bottle of carbon tetrachloride in mysterious circumstances—a death that has eerie echoes of Kenneth Williams's own. He drank from a bottle labelled Gees Linctus but which actually contained poison, and the coroner recorded a verdict of death by misadventure. ... Many, perhaps Kenneth included, believed it was suicide.
- http://www.mailonsunday.co.uk/femail/article-1082192/Did-Kenneth-Williams-poison-father.html "Did Kenneth Williams poison his father?" Daily Mail, 31 October 2008
- Radio Times (London edition) 23–29 July 1977
- Overdose http://www.britmovie.co.uk/actors/w/004.html Retrieved 8 October 2007
- The Guardian 17 June 1988 "Open verdict recorded on Williams"
- Christopher Stevens, Born Brilliant: The Life Of Kenneth Williams, John Murray, 2010 isbn = 1-84854-195-3
- http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=18712 "Kenneth Williams" Find a Grave