Davies was born in Johnstone in Scotland to Scottish parents. For 4 years his family lived in Dumfries until Davies was aged 11. Davies has frequently quoted his boyhood hero as being football centre-forward, Billy Houliston, of Davies' then local team, Queen of the South
His family moved to Carlisle 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. However 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 that was quickly made into a film. 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 and 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 he wrote what is widely regarded as one of the best ever books about football, The Glory Game, a behind the scenes portrait of Tottenham Hotspur. Davies also wrote a wry 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.
Davies has also written a biography of the hill walker Alfred Wainwright, and many works about the topography and history of the Lake District.
In children's literature, he has written the "Ossie", "Flossie Teacake" and "Snotty Bumstead" series of novels.
As a ghostwriter, he has worked on the autobiographies of footballers Wayne Rooney, Paul Gascoigne and Dwight Yorke. The Wayne Rooney biography led to a successful libel action in 2008 by David Moyes, the manager of his former club, Everton. He has also ghostwritten politician John Prescott's 2008 autobiography, Prezza, My Story: Pulling no Punches.
He writes a football column for the New Statesman magazine which is written in his trademark humorous, irreverent tone. 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