Tom Baker was born in Scotland Road, Liverpool. His parents were working class Liverpudlians. Baker left school at 15 to become a Roman Catholic monk and remained in the monastic life for six years, but left after losing his faith, and did his National Service in the Royal Army Medical Corps, serving for two years from 1955 until 1957. 

In the late 1960s and early 1970s Baker was part of Laurence Olivier's National Theatre company, and got his first big film break in 1971 with the role of Rasputin in the film Nicholas and Alexandra. for which he was nominated for two Golden Globe Awards, one for Best Actor in a Supporting Role and another for Best Newcomer. 

In 1974, Baker was cast by Barry Letts to take over the role of the Doctor. Letts was convinced he was right for the part after seeing his performance in The Golden Voyage of Sinbad. He played the Doctor for seven seasons, making him the longest-serving actor in the part so far. T

he manifestation played by Tom Baker (1974-1981) is often regarded as the most popular of the Doctors. In polls conducted by Doctor Who Magazine, Baker has lost the "Best Doctor" category only three times: Once to Sylvester McCoy in 1990, and twice to David Tennant in 2006 and 2009.

Baker returned to the role for a series of five audio dramas for BBC Audio, co-starring Richard Franklin as Captain Mike Yates, which were released in September 1979. He has also played the Fourth Doctor for a series of plays for Big Finish Productions. 

In 2001 Baker was cast as the narrator of Little Britain on BBC Radio 4, and remained in the role when it transferred to television. From 2000 to 2005 Tom voiced the character Max Bear in the Channel 4 (UK) Max Bear Productions animated series. 

More recently, he voiced the role of the villain ZeeBad in the 2005 computer-animated film version of The Magic Roundabout. Baker narrates the children's computer animation series The Beeps which is shown on Five's Milkshake! as well as narrating Tales of Aesop on BBC, a television series based on Aesop's Fables with puppet animation. Baker played Puddleglum, a "marsh-wiggle", in the BBC adaptation of C.S. Lewis' The Silver Chair. 

He also portrayed Sherlock Holmes in a four part BBC miniseries version of The Hound of the Baskervilles in 1982. Baker made a memorable appearance in Blackadder II, as a wild-eyed sea captain who advocated drinking one's own urine as a survival tactic. 

In a 2005 survey of British adults, Baker's voice was found to be the fourth most recognisable after the Queen, Tony Blair and Margaret Thatcher. During the first three months of 2006, his voice was used by BT for spoken delivery of text messages to landline phones. He recorded 11,593 phrases, containing every sound in the English language, for use by the text-to-speech service. His voice may be heard at London's Natural History Museum narrating commentary to some of the exhibits that support Darwin's theory of natural selection.

Baker's autobiography, Who on Earth is Tom Baker? , was published in 1997. He has also written a short fairytale-style novel, The Boy Who Kicked Pigs, which is subtitled "A Grotesque Masterpiece".

Baker's first marriage in 1961 was to Anna Wheatcroft. They had two sons Daniel and Piers, but divorced in 1966 and Baker lost contact with his sons until a chance meeting with Piers in a pub in New Zealand allowing them to renew their relationship. In December 1980 he married Lalla Ward although, the marriage lasted only 16 months. In 1986, Baker married for a third time, this time to Sue Jerrard, who had been an assistant editor on Doctor Who.