Mantell was born in Romiley, then part of Cheshire, the second son of Francis Christopher Knight Mantell and Elsie, née Caton. His younger brother was actor, Knight Mantell. He attended Manchester Grammar School and studied law at the University of Manchester. He married (Anne) Shirley Cogger in 1960 after National Service in the Royal Air Force between 1958 and 1961, he was called to the bar by Gray's Inn and later served at the chambers of Sir Patrick Russell in Manchester. He practised on the Northern Circuit and was present at the trials of the Birmingham Six, Trevor Hardy and Terry Clark. Between 1982 and 1985 Mantell was a Supreme Court judge in Hong Kong, returning to Britain as a judge on the Western Circuit. In 1990 he was appointed to the High Court and knighted.
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