Warner was born in Manchester, England and trained at RADA, before making an impact with the Royal Shakespeare Company, where his portrayal of Hamlet quickly made him famous. In 1963, he made his film debut in Tom Jones, and in 1965 starred as King Henry VI of England in the BBC production of Shakespeare's "Wars of the Roses" cycle. Another major step in his career was the leading role in Morgan: A Suitable Case For Treatment (1966), which established his reputation for playing slightly off-the-wall characters.
Since then, he has specialised in playing villains, in films such as The Thirty-Nine Steps (1978), Time After Time (1979), and Tron (1982), and television series such as Batman: The Animated Series. He has appeared in hit movies such as Star Trek VI: The Undiscovered Country, Titanic, Scream 2, and more recently in independent television's adaptation of the Hornblower series. He also continues to play classical roles.