Mantle was educated at Kimbolton School, Cambridgeshire between 1970 and 1975 and at the Royal Academy of Dramatic Art (RADA) between 1978 and 1980. He appeared in 11 productions of the National Youth Theatre in five seasons between 1974 and 1978, and began carving a successful career as a stage actor in the 1980s, alongside various television roles. In 1984 he was nominated for an Olivier Award and was joint Best Newcomer in the Plays and Players Awards for his performance as Lennie in Of Mice and Men. That year he was cast as Little John in Robin of Sherwood. 

Clive played general surgeon Mike Barratt in the BBC hospital drama series Casualty and Holby City in the 1990s, and Little John in the 1980s fantasy series Robin of Sherwood. He returned to Casualty in 2016 as Mike Barratt for the show’s 30th anniversary. More recently, Mantle has become known for his roles on television as Lord Greatjon Umber in HBO’s Game of Thrones and as Tony Curry, Ollie’s (Will Mellor’s) father, in the BBC’s White Van Man. He has also played several minor roles in other Hollywood films such as White Hunter Black Heart (1990) and Alien 3 (1992).

On stage, Mantle has appeared in plays such as Coming Clean, A Streetcar Named Desire, and Educating Rita, and has continued his successful stage career since departing from Holby City. In 2003 he appeared in Rattle of a Simple Man; in 2006 he played the part of The Narrator in The Rocky Horror Show; and over the Christmas and New Year period of 2007–2008, he portrayed the villain Abanazer in a pantomime production of Aladdin at the Theatre Royal in Bath. In 2010 he portrayed comedian Tommy Cooper in the stage entertainment show Jus’ Like That! A Night Out with Tommy Cooper.

Clive will be reprising his role as Captain Shaw in Iolanthe with the ENO, a role he first played in the 2018 production.