Pelka was born in Dewsbury, Yorkshire to an actress mother and a civil engineer father. His mother is Irish, and his father is Polish. Valentine is well known for his guest appearance on Highlander: The Series as the evil Immortal Kronos, an enemy of Duncan MacLeod and former acolyte of Methos in the episodes "Comes a Horseman", "Revelations 6:8", "Archangel", and "Not To Be".

He has starred in some TV series, including the 1986 TV series Crossbow, as Roland; his other TV roles include Family Affairs in 1999 as Simon Thornton, and in the 2000 television series Queen Of Swords as Col. Luis Montoya and the 2000 series Life Force as Richard Webster.

Valentine has starred in films; his best-known feature film is the 1995 movie First Knight, in which he played Sir Patrise, and his most recent role is in the 2005 movie The Velvet Side of Hell.

Valentine has made guest appearances on many TV shows; some of those appearances range from Robin of Sherwood, Boon, Campion, Hustle, Egypt, Highlander: The Series and Cadfael (TV series).

His sister is actress Kazia Pelka who became a leading player in the soap opera Family Affairs he had previously appeared in.