Warner was a book salesman before training for the stage. He made his debut in the 1962 Royal Shakespeare Company production of "A Midsummer Night's Dream" as Snout and in 1965 and 1966 won over all critics by playing "Hamlet" in the afternoons and Andrew Aguecheek in "Twelfth Night" in the evenings at Stratford-upon-Avon. Warner's stage work tapered off in the 70s as film and TV roles began to take more of his time. He made his screen debut as the sleazy Blifil in Tony Richardson's raucous "Tom Jones" (1963), but it was "Morgan - A Suitable Case for Treatment/Morgan!" that made him a viable screen actor. He went on to give notable performances as Torvald opposite Jane Fonda's Nora in Joseph Losey's 1973 adaptation of "A Doll's House" (which played on American TV), and was chilling as Jack the Ripper transported to present-day San Francisco in "Time After Time" (1979). Warner's output in the 80s and 90s often included large-scale spectacle films, such as "Tron" (1982) and both "Star Trek V: The Final Frontier" (1989) and "Star Trek VI: The Undiscovered Country" (1991). In the latter two, he was the Klingon chancellor who says he wants to negotiate a peace. Warner even played the affable scientist who discovers the secret of the crime fighters in "Teenage Mutant Ninja Turtles II: The Secret of the Ooze" (1991).
Warner has compiled an extensive TV resume, gaining steam in the 70s with the British series "Clouds of Glory" and coming to the attention of American audiences as Reinhard Heydrich, the general in charge of the final solution, who promotes Michael Moriarty, in the NBC miniseries "Holocaust" (1978). In 1979, he was a bachelor who flirts with Susan Saint James and survives the demise of the "S.O.S. Titanic" (ABC), and in 1981, Warner won a Best Supporting Actor Emmy for his work as a waspish Roman in the ABC miniseries "Masada". He was Bob Cratchit to George C. Scott's Scrooge in CBS' 1984 rendition of "A Christmas Carol" and again played Heydrich in "SS: Portrait in Evil" (NBC, 1985). Warner starred in the 1984 British series "Nancy Astor" (PBS) about the American socialite who came to Britain and stayed. In 1996, Warner was the scientist doing bizarre experiments in "Naked Souls" and Dr. Botkin who tends to "Rasputin", both for HBO.
1997 proved a banner year for Warner, who not only narrated "Pooh's Grand Adventure: The Search for Christopher Robin", but also landed parts in "Scream 2" and, more significantly, James Cameron's blockbuster "Titanic". He portrayed drama teacher Gus Gold in the former and took on a villainous part in the latter that would have drawn hisses from an old-fashioned melodrama crowd, that of Cal Hockley's (Billy Zane) ruthlessly loyal valet Spice Lovejoy. Warner became a series regular, playing "The Man" who gives out assignments in The WB's "Three" (1998), a sort of "Mod Squad" for the 90s. He also appeared in "The Leading Man" (1997), starring Jon Bon Jovi.