In 1971, Baker got his first break with the role of Rasputin in the film Nicholas and Alexandra. He also appeared nude in Pier Paolo Pasolini's version of Chaucer's The Canterbury Tales (I Racconti di Canterbury), in 1972, as a younger husband of the Wife of Bath.
In 1974, 40-year-old Baker took on the role of the Doctor when Jon Pertwee left. He was cast because of his performance in The Golden Voyage of Sinbad. Baker was working on a construction site at the time. He was dubbed "Boiler Suit Tom" by the media, as he had been supplied for a press conference with some old studio set clothes to replace his modest garments.
As the Doctor, his eccentric dress and speech — particularly his long scarf and jelly babies — made him immediately recognisable. Baker played the Doctor for seven consecutive seasons, the longest-serving actor in the part on-screen. Baker suggested many aspects of the Doctor's personality. The scarf came by accident: James Acheson, the costume designer, had provided more wool than necessary to the knitter, Begonia Pope, who knitted all she was given; Baker suggested he wear the ridiculously long scarf.
In October, 1980, the BBC announced just before the serial Full Circle that Baker was leaving at the end of the 1981 season. Baker's departure was based partly on his feeling that he could not develop his role further, and partly on the new producer, John Nathan-Turner's dislike of Baker's portrayal. Baker's last regular appearance as the Doctor was on 21 March 1981 at the conclusion of the story Logopolis.