Mortimer was born in Hampstead, London, the only child of Kathleen May (née Smith) and Clifford Mortimer, a barrister who became blind in 1936, when he banged his head on a tree branch, but still pursued his career. His father's loss of sight sight was not acknowledged openly by the family.

Mortimer was educated at the Dragon School and Harrow where he joined the Communist Party forming a one member cell. Originally Mortimer intended to be an actor, his lead role in the Dragon's 1937 production of Richard II, gained glowing reviews in The Draconian, and then a writer, but his father persuaded him against it advising: "My dear boy, have some consideration for your unfortunate wife ... [the law] gets you out of the house."

At seventeen, he went up to Brasenose College, Oxford where he read law, though he was actually based at Christ Church because the Brasenose buildings had been requisitioned for the war effort. In July 1942, at the end of his second year, Mortimer was asked to leave Oxford by the Dean of Christ Church, after letters to a Bradfield sixth-former, Quentin Edwards, later a QC, were discovered by the young man's housemaster.

Mortimer was classified as medically unfit for military service in World War II, with weak eyes and doubtful lungs. He worked for the Crown Film Unit, writing scripts for propaganda documentaries. "I lived in London and went on journeys in blacked-out trains to factories and coal-mines and military and air force installations. For the first and, in fact, the only time in my life I was, thanks to Laurie Lee, earning my living entirely as a writer. If I have knocked the documentary ideal, I would not wish to sound ungrateful to the Crown Film Unit. I was given great and welcome opportunities to write dialogue, construct scenes and try and turn ideas into some kind of visual drama." He based his first novel Charade on his experiences with the Crown Film Unit.

Mortimer made his radio debut as a dramatist in 1955 with his adaptation of his own novel, Like Men Betrayed for the BBC Light Programme. But he made his debut as an original playwright with The Dock Brief, starring Michael Hordern as a hapless barrister, first broadcast in 1957 on BBC Radio's Third Programme, later televised with the same cast and subsequently presented in a double bill with What Shall We Tell Caroline? at the Lyric Hammersmith in April 1958, before transferring to the Garrick Theatre. It was revived by Christopher Morahan in 2007 as part of a touring double bill, Legal Fictions.

His play, A Voyage Round My Father, given its first radio broadcast in 1963, is autobiographical, recounting his experiences as a young barrister and his relationship with his blind father. It was memorably televised by BBC Television in 1969 with Mark Dignam in the title role. In a slightly longer version the play later became a stage success (first at Greenwich Theatre in 1979 with Dignam, then a year later at the Theatre Royal Haymarket, now starring Alec Guinness). In 1981 it was remade by Thames Television with Sir Laurence Olivier as the father and Alan Bates as young Mortimer.

In 1965, he and his wife wrote the screen play for the Otto Preminger film Bunny Lake is Missing.

Mortimer was called to the Bar (Inner Temple) in 1948, at the age of 25. His early career consisted of testamentary and divorce work, but on taking silk in 1966 he began to undertake work in criminal law. His highest profile, though, came from cases relating to claims of obscenity which according to Mortimer were "alleged to be testing the frontiers of tolerance".

Though sometimes thought to have been involved in the Lady Chatterley's Lover obscenity trial defence team, he successfully defended publishers John Calder and Marion Boyars in their 1968 appeal against their conviction for publishing Hubert Selby, Jr.'s Last Exit to Brooklyn. Mortimer fulfilled the same role three years later, this time unsuccessfully, for Richard Handyside, the English publisher of The Little Red Schoolbook.

Mortimer was defence counsel at the Oz conspiracy trial later in 1971. In 1976 he defended Gay News editor Denis Lemon (Whitehouse v. Lemon) for the publication of James Kirkup's "The Love that Dares to Speak its Name" against charges of blasphemous libel; Lemon was convicted with a suspended prison sentence, later overturned on appeal.His defence of Virgin Records in the 1977 obscenity hearing for their use of the word bollocks in the title of the Sex Pistols album Never Mind The Bollocks, and the manager of the Nottingham branch of the Virgin record shop chain for the record's display in a window and its sale, led to the defendants being found not guilty.

Mortimer retired from the bar in 1984.

Mortimer is best remembered for creating a barrister named Horace Rumpole, whose speciality is defending those accused of crime in London's Old Bailey. Mortimer created Rumpole for Rumpole of the Bailey, based on a chance Court encounter with James Burge QC, as a 1975 contribution to the BBCs Play For Today anthology series. Although not Mortimer's first choice of actor, Leo McKern played the character with gusto proving popular, and the idea was developed into a series Rumpole of the Bailey for Thames Television and a series of books (all written by Mortimer). In September-October 2003, BBC Radio 4 broadcast four new 45-minute Rumpole dramatizations by Mortimer starring Timothy West in the title role. He also dramatised many of the real-life cases of the barrister Edward Marshall-Hall in a radio series starring ex-Doctor Who star Tom Baker.

Mortimer was credited with the adaptation of Evelyn Waugh's Brideshead Revisited for Granada Television in 1981. However, Graham Lord's unofficial biography, John Mortimer: The Devil's Advocate, revealed in 2005 that none of Mortimer's submitted scripts had in fact been used and that the screenplay was actually written by the series producer and director. Mortimer adapted John Fowles' The Ebony Tower, starring Laurence Olivier for Granada in 1984.

In 1986, his description of what he saw as Britain's descent into the viciousness of Thatcherism — Paradise Postponed — was televised, in an adaptation from his own novel.

He also wrote the script, based on the autobiography of Franco Zeffirelli, for the 1999 film Tea with Mussolini, directed by Zeffirelli and starring Joan Plowright, Cher, Judi Dench, Maggie Smith and Lily Tomlin. From 2004, Mortimer worked as a consultant for the politico-legal US comedy television show Boston Legal.

He developed his career as a dramatist by rising early to write before attending court, and his work in total includes over fifty books, plays, and scripts.

Mortimer married Penelope Fletcher (he was her second husband), later better known as Penelope Mortimer, in 1949 and had a son and a daughter by her, Sally Silverman and Jeremy Mortimer. The unstable marriage inspired work by both writers, but Penelope's novel, The Pumpkin Eater (1962), later filmed, is the best known. The couple divorced in 1971 and he married Penelope Gollop in 1972. They had two daughters, Emily Mortimer, and Rosie Mortimer. He lived with his second wife in the village of Turville Heath in Buckinghamshire. The split with his first wife had been bitter, but they were on friendly terms by the time of her death in 1999.

In September 2004 Tim Walker, The Sunday Telegraph's Mandrake diarist, revealed the existence of a second son, Ross Bentley, conceived during a secret affair Mortimer pursued with the English actress Wendy Craig more than 40 years earlier, and born in November 1961. Craig and Mortimer had met when the actress had been cast playing a pregnant woman in Mortimer's first full-length west end play, The Wrong Side of the Park. Ross Bentley was raised by Craig and her husband, Jack Bentley, the show business writer and musician. In Mortimer's memoirs, Clinging to the Wreckage, he wrote of "enjoying my mid-thirties and all the pleasures which come to a young writer."

Awarded the CBE in 1986, he was knighted in 1998.

Mortimer died on 16 January 2009, aged 85, after a long illness. He had suffered a stroke in October 2008.

John Mortimer was a patron of the Burma Campaign UK, the London-based group campaigning for human rights and democracy in Burma, and was the president of the Royal Court Theatre having been the chairman of its board from 1990 to 2000. Earlier, he was on the board of the National Theatre from 1968 to 1988.