Frewer was born in Washington, D.C., the son of Gillian Anne (née German) and Frederick Charlesley Frewer, a Canadian naval officer. He was raised in Peterborough, Ontario, and trained at the famous Bristol Old Vic Theatre School, graduating from its three-year acting course in 1980.

In 2009 Frewer played the retired villain Moloch the Mystic in the film adaptation of the comic book series Watchmen, and appears as the White Knight in the December 2009 Syfy two-part miniseries Alice, based upon Alice's Adventures in Wonderland. Frewer starred as Pestilence in two episodes of Season 5 of Supernatural.

He holds dual citizenship in Canada and the United States. When not on location, he, his wife, Amanda Hillwood, and their daughter divide their time between Malibu, California, and the Gatineau Hills of Quebec.