His family moved to Carlisle in England when Davies was 11 and he attended the Creighton School in the city. Davies lived in Carlisle until he moved to study at university. During this time his father, who was a former RAF pay clerk, developed multiple sclerosis and had to retire on medical grounds from a civil service career. Davies joined the sixth form at Carlisle Grammar School and was awarded a place at University College, Durham to read for an honours degree in History, but after his first year he switched to a general arts course. He gained his first writing experience as a student, contributing to the university newspaper, Palatinate. After completing his degree course he stayed on at Durham for another year to gain a teaching diploma.
After he left university, Davies worked as a journalist, and in 1965 he wrote the novel Here We Go Round the Mulberry Bush, which was made into a film of the same name. He raised the idea of a biography of the Beatles with Paul McCartney when he met him to discuss the possibility of providing the theme song for the film. McCartney liked the idea of the book because inaccurate information had been published about the group, and he advised him to obtain the approval of Brian Epstein. He agreed to it and the resulting authorised biography, The Beatles, was published in 1968.
John Lennon mentioned in his 1971 Rolling Stone interview that he considered the book "bullshit", though Lennon at the time was vigorously debunking the Beatle myth and anyone who had helped to create it.
In 1972 Davies wrote a book about football, The Glory Game, a behind-the-scenes portrait of Tottenham Hotspur. Davies also wrote a column about his daily life in Punch called "Father's Day", presenting himself as a harried paterfamilias. In 1974 he was sent by the Sunday Times to look at a comprehensive school in action. He wrote three articles and then stayed on at the school – Creighton School in Muswell Hill, North London, now part of Fortismere School – to watch and study through a year in its life. The result was a book, the Creighton Report, published in 1976.
He writes a football column for the New Statesman,. A compilation of these articles was released as a book, The Fan, in 2005 by Pomona Press. Davies writes "Confessions of a Collector" in The Guardian's Weekend colour magazine. He has written a book about his collections with the same title.