He did his national service in the RAF – "it confirmed my natural pacifism" – then went to Christ Church, Oxford, where he became editor of the student weekly Isis. He wrote poems in the disciplined forms of the Movement, won prizes, published a pamphlet. Equipped for journalism, he joined The Oxford Mail in 1955 and then the Evening Standard's Londoner's Diary, until 1963. Later he became a television critic and wrote about pop music; the Sunday Times fired him for reviewing Peter Watkins' embargoed anti-nuclear film The War Game. But he had set his sights on becoming a writer and, with a small legacy from his mother, left journalism, and wrote a television play and his first novel If You See Me Comin' (1962), a bluesy, chilling account of an execution in a glum provincial city. Like all of his portrayals of injustice, it is coloured by a barely suppressed sense of terror.
Meanwhile he was reading his poems in the burgeoning British movement of performed poetry. I met him in 1962 at one such reading, for Arnold Wesker's Centre 42 arts festivals for working- class audiences. He leapt on stage in a many coloured coat like a Blakean challenger and a rock'n'roll hero. He had fine music-hall timing, and a gravity under all the quickfire jokes and patter. He began to bring out a steady flow of poetry volumes, from Out Loud (1968) to Tell Me Lies (it will be published next year) – 15 books of free, syncopated, carnivalesque poems about love, war, children, politicians, pleasure, music. 'He breathed in air/He breathed out light/ Charlie Parker was my delight.'
With their zany Ralph Steadman covers, these books quickened the reader's imagination. Opening a new one was like an invitation to a party where the dancing never stopped. "He has the innocence of his own experience," said Ted Hughes; "the British Mayakovsky," said Kenneth Tynan; "the kind of tenderness sometimes to be found between animals," wrote John Berger.
To Whom It May Concern, a riveting poem against bombs and cenotaphs and the Vietnam war, with which he stirred a capacity audience in Mike Horovitz's pioneering Poetry Olympics at the Albert Hall in 1965, has lasted through the too many wars since: a durable counting-rhyme to a rhythm and blues beat.
The 1960s brought two life-changing events for Mitchell. He met the actor Celia Hewitt, working for Tynan on ITV's arts programme Tempo. She was his partner for the last 47 years. He also met Jeremy Brooks, literary manager of the Royal Shakespeare Company. He showed his lyrics to Peter Brook, who was looking for someone to adapt a literal translation of Peter Weiss's play The Marat/Sade. Brook jumped, and Adrian worked to the bone to meet a rehearsal deadline and make a glittering, dark text for this 1964 kaleidoscopic play about revolution on the street and in the head.
The encounter with Brook was an upheaval, and Adrian went on to join Brook's team for the collectively authored US (1966), about the Vietnam war, created out of 14 weeks rehearsal and no pre-existing script. His song lyrics, including Tell Me Lies About Vietnam already famous in the anti-war movement, sharpened the ironies of the show; his involvement in heated group debates about the direction of the show was critical, gentle and firm. My own favourite as a team member was Barry Bondhus, a talking blues about a father who dumped human excrement into army filing cabinets. It showed a love of Adrian's true America, the land of Whitman, Guthrie and Ginsberg, which marked him out from simplistic anti-Americanism.
From a play about Blake, Tyger, (1971) for Olivier's National Theatre, a time-travelling musical about a visionary 18th-century poet in today's fallen times, with music by long-term collaborator Mike Westbrook, to a version of Pushkin's Boris Godunov for the RSC (due next year) Adrian wrote more than 30 plays, operas, children's plays, classic adaptations. Some were for major companies, many more for the alternative British theatre, from regional playhouses to site-specific groups such as John Fox's Welfare State. The Liverpool Everyman in its heyday staged his Mind Your Head, a phantasmagorical bus journey. His Pied Piper ran at the National for three years, and his The Lion, the Witch and the Wardrobe became a perennial favourite at the RSC. He made a Beatrix Potter trilogy for the Unicorn Theatre for Children, adapted Spanish classics and Gogol's The Government Inspector for the National, and wrote songs for Peter Hall's version of Orwell's Animal Farm. In 2006, for the Woodcraft Folk Global Peace Village, he staged The Fear Engine in a vast field, a panorama of threatening world politics for a cast of hundreds of young people.
The musical nature of Adrian's imagination led him to work with a cavalcade of composers and performers: Andy Roberts, Richard Peaslee, Steve McNeff, Dominic Muldowney, Andrew Dixon and Stephen Warbeck. His influence radiated widely, not least to generations of teachers, who used his poems with children in schools.
Last week he rang me. He sounded better than during his last three months of illness. "Can I read you this poem?" he asked. He did. It was a celebration. Next night he died. But this poem, and the poems and the plays and the politics – he went to Faslane on the anti-Trident demonstration and got arrested – will last. He is survived by Celia, two sons, three daughters and nine grandchildren.