His television work began with an episode during the second series of Doctor Who in 1964, for which he was paid £30. Pickup worked with Laurence Olivier at the Royal National Theatre, most notably in Three Sisters and Long Day's Journey Into Night. In 1973, he starred in the BBC drama series The Dragon's Opponent, playing a World War II bomb disposal expert and also appeared in The Day of the Jackal. Pickup played Cranmer, Archbishop of Canterbury in the 1979 BBC & Time-Life film of Shakespeare's Henry VIII. He played Lt. Harford in Zulu Dawn in 1979, portrayed Igor Stravinsky in Nijinsky in 1980, Prince John in Ivanhoe in 1982, and in 1983 he appeared opposite Penelope Keith in Moving and as Friedrich Nietzsche in Wagner, in 1988 in the BBC miniseries The Lion, the Witch and the Wardrobe as the voice of Aslan, and in 1990 he starred in the short lived sit-com, Not with a Bang. More modern roles have included parts in Hornblower, Hustle, Foyle's War, Midsomer Murders, Waking the Dead, The Bill, Silent Witness, Sherlock Holmes, and Inspector Morse. In 2004, he appeared in the movie Secret Passage alongside John Turturro. Pickup played a regular part in the BBC sitcom The Worst Week of My Life. His most recent appearance was in Holby City as Lord Charles Byrne.
Pickup gave a highly acclaimed performance as a decayed Russian aristocrat in the BBC series Fortunes of War, based on a work by Olivia Manning. He also provided the voice for Aslan in the BBC's adaptation of the Chronicles of Narnia and starred opposite Judi Dench in the 1989 Channel 4 serial Behaving Badly.