Mantle was born in Barnet, Hertfordshire. He was a chorister in St. John's College Choir, Cambridge for four years, and a member of the National Youth Theatre for five years, appearing in a total of eleven productions.
He trained at RADA and has worked steadily in films, plays and television, most notably in the cult 1980s fantasy series Robin of Sherwood, where, at 6' 5½" (1.97 m), he was cast as Little John. Mantle is also known for his role as general surgeon Mr. Mike Barrett in the TV series Casualty and later Holby City. Incidentally, he also played the brother of a patient in series 3 of Casualty. He played a minor character in the film Alien 3 in 1993. In 2006 he guest starred in the Doctor Who audio drama The Settling by Big Finish Productions.
He also portrayed Simon Horton, younger brother of David Horton in the British sitcom The Vicar of Dibley. He appeared as an electric meter reader in the One Foot in the Grave episode 'The Return of the Speckled Band', as well as an irate husband whose wife is chatted up by Richie in the first episode of Bottom,Smells.
His movie debut role was as the first Nuclear Man in Superman IV: The Quest For Peace (1987), but the role was left on the cutting room floor.
Some of his deleted footage from Superman IV: The Quest for Peace was released on DVD in a Special Edition as part of Superman Ultimate Collector's Edition in November 2006. The footage was included as "Additional Scenes" but was not reincorporated into the main feature.
Clive was awarded the Laurence Olivier Theatre Award in 1986 (1985 season) for Most Promising Newcomer for his performance in Of Mice and Men.
His cousin is actor John Hallam. Mantle is a fan of the Chelsea Football Club.