Warner had a long and illustrious acting career appearing in films such as The Omen, Time Bandits, Tron, The Lost World, Titanic and Planet of the Apes. 

He appeared in the Star Trek films The Final Frontier and The Undiscovered Country as well as playing Gul Madred in Star Trek: The Next Generation. Warner lent his vocal talents to Doctor Who providing the voice of Lord Azlok in the animated adventure Dreamland, first shown in 2009. He also appeared in many audio plays for Big Finish Productions, where he played an alternative version of The Doctor.

In 1981, he won an Emmy Award for Outstanding Supporting Actor in a Miniseries or Special for his portrayal of Pomponius Falco in the television miniseries Masada.