Film and television
In 1963, he made his film debut as the villainous Blifil in Tom Jones, and in 1965, starred as Henry VI in the BBC television version of the RSC's The Wars of the Roses cycle of Shakespeare's history plays. He starred alongside Bob Dylan in the 1963 play Madhouse on Castle Street. A major step in his career was the leading role in Morgan: A Suitable Case for Treatment (1966), opposite Vanessa Redgrave, which established his reputation for playing slightly off-the-wall characters. He also appeared as Konstantin Treplev in Sidney Lumet's 1968 adaptation of Anton Chekhov's The Sea Gull and starred alongside Jason Robards and Stella Stevens as Reverend Joshua Duncan Sloane in Sam Peckinpah's The Ballad of Cable Hogue.
In horror films, he appeared in one of the stories of From Beyond the Grave, opposite Gregory Peck in The Omen (1976), as the ill-fated photojournalist Keith Jennings, and the 1979 thriller Nightwing. He also starred in cult classic Waxwork (1988), and featured alongside a young Viggo Mortensen in the 1990 film Tripwire.
He often played villains, in films such as The Thirty Nine Steps (1978), Time After Time (1979), Time Bandits (1981), Tron (1982), Hanna's War (1988). Warner's voice acting roles in television include Ra's al Ghul in Batman: The Animated Series, Herbert Landon in Spider-Man: The Animated Series, Alpha in Men in Black: The Series, the Archmage in Disney's Gargoyles, and the Lobe in Freakazoid!. He was also cast against type as Henry Niles in Straw Dogs (1971) and as Bob Cratchit in the 1984 telefilm A Christmas Carol starring George C. Scott as Scrooge. In addition, he played German SS Obergruppenführer Reinhard Heydrich both in the film Hitler's SS: Portrait in Evil, and the television miniseries Holocaust; as sinister millionaire Amos Hackshaw in HBO's original 1991 film Cast a Deadly Spell.
In 1981, Warner received an Emmy Award for Outstanding Supporting Actor in a Miniseries or Special for Masada as Pomponius Falco. In 1988, he appeared in the Danny Huston film Mr. North.
He subsequently appeared in films such as Star Trek V: The Final Frontier (1989), Star Trek VI: The Undiscovered Country, Avatar (known as Matrix Hunter in the US), Teenage Mutant Ninja Turtles II: The Secret of the Ooze (1991), Titanic (the third time he appeared in a film that is about or includes reference to RMS Titanic) and Scream 2. In 2001, he played Captain James Sawyer in two episodes of A&E's adaptation of C.S. Forester's Hornblower series. He appeared in three episodes of the second season of Twin Peaks (1991) as "Thomas Eckhardt". He also continued to play classical roles. In "Chain of Command", an episode of Star Trek: The Next Generation, he was a Cardassian interrogator. He based his portrayal on the evil "re-educator" from 1984. He appeared in Murder, She Wrote in 1993 as a Hong Kong-based detective. His less-spectacular roles included a double-role in the low-budget fantasy Quest of the Delta Knights (1993) which was eventually spoofed on Mystery Science Theater 3000. He also played Admiral Tolwyn in the film version of Wing Commander.
Warner's sympathetic side had been evident in Sam Peckinpah's Cross of Iron (1977), where he portrayed Captain Kiesel. Other "nice guy" roles include in Ken Russel's William and Dorothy (1978), portraying the poet William Wordsworth, the charismatic "Aldous Gajic" in "Grail", a first season (1994) episode of Babylon 5 and "Chancellor Gorkon" in Star Trek VI: The Undiscovered Country (1991). In an episode of Lois & Clark: The New Adventures of Superman, he played Superman's father Jor-El, who appeared to his son through holographic recordings. Warner also played "ambiguous nice guys" such as vampire bat exterminator Philip Payne in 1979's Nightwing; and Dr. Richard Madden in 1994's Necronomicon: Book of the Dead. In Seven Servants by Daryush Shokof, he co-starred with Anthony Quinn in 1996.
Another 'sympathetic' role was in 2013, when he played Professor Grisenko in the Doctor Who episode "Cold War" in which he battled a revived Ice Warrior and struck up a rapport with the Doctor's companion Clara Oswald. Warner also appeared in the second series of the Sky 1 comedy-drama Mad Dogs, and starred in two 2014 episodes of the horror series Penny Dreadful as Abraham Van Helsing.
Warner contributed "Sonnet 25" to the 2002 compilation album When Love Speaks, which consists of Shakespearean sonnets and play excerpts as interpreted by famous actors and musicians. He performed in many audio plays for Big Finish Productions, starring in the Doctor Who Unbound play Sympathy for the Devil (2003) as an alternative version of the Third Doctor who was exiled to Earth in 1997 instead of the 1970s after his trial in The War Games. Warner also appeared in a series of plays based on ITV's Sapphire & Steel as Steel. He reprised his incarnation of the Doctor in a Doctor Who Unbound sequel, Masters of War (2008), where he travelled in the TARDIS to Skaro with Brigadier Lethbridge-Stewart. In 2007, Warner guest starred as Isaac Newton in the Doctor Who audio drama Circular Time and as Cuthbert in four of the seven stories in the second Fourth Doctor series. He also guest starred in the BBC Radio 4 science fiction comedy Nebulous (2005) as Professor Nebulous' arch-enemy Dr. Joseph Klench. In all these productions, Warner worked with writer and comedian Mark Gatiss of the League of Gentlemen, and plays a guest role in the League's 2005 feature film The League of Gentlemen's Apocalypse. He also performed in radio plays for the American companies L.A. Theatre Works and the Hollywood Theater of the Ear. In 2005, Warner read a new adaptation of Oliver Twist for BBC Radio 2 (adapted by Neville Teller and directed by Neil Gardner). In 2008, he guest-starred as Mycroft Holmes in the Bernice Summerfield audio play The Adventure of the Diogenes Damsel. In 2009, he was the voice of Lord Azlok of the Viperox, an insectoid alien race in the animated Doctor Who serial "Dreamland". In 2016, he returned as his alternate Doctor in a series of audios where his Doctor briefly travels to the 'prime' universe and enlists the Seventh Doctor's companion Benny Summerfield (Lisa Bowerman) to try and help him save his universe. Warner's Doctor continued his travels with Benny in a second series of audios released in 2017. Warner ended up doing five seasons of The New Adventures of Bernice Summerfield playing his alternate Doctor. Shortly before his death in 2022, it was revealed Warner would return as his alternate Doctor as part of Finish's celebration of the 60th anniversary and would share scenes with Christopher Eccleston, who appeared as the Ninth Doctor.
He also contributed voice acting to a number of video games, notably playing the villain Jon Irenicus in Baldur's Gate II: Shadows of Amn and Morpheus in Fallout. Warner did voice work on the short-lived FOX animated series Toonsylvania as Dr. Vic Frankenstein. He was also the first voice of the demon Nergal from The Grim Adventures of Billy & Mandy, but was later replaced by Martin Jarvis. Warner narrated the Disney direct-to-video Pooh's Grand Adventure: The Search for Christopher Robin.
In March 2010, it was announced that Warner would be joining the cast of the Dark Shadows audio drama miniseries Kingdom of the Dead.