Often known as Billy, he was educated at home and at Imperial Service College. After training as a jockey, and boxing, he entered the theatre in 1924. The first of more than sixty film appearances was Say It With Music in 1932. Hartnell usually played comic characters, until 1944 with the robust role of sergeant Ned Fletcher in The Way Ahead. From then on, he played mainly policemen, soldiers, and thugs, like Dallow in Brighton Rock.
He appeared first on television in The Army Game from 1957-61, and in 1963 gave up movies and took the lead in Doctor Who, for which he is now most widely known.
He was a tough person to work with, according to the documentation about him. His poor health - arteriosclerosis brought on by years of heavy drinking - as well as poor relations with the new production team forced him to leave Doctor Who in 1966, although he reprised the role in the 1973 story The Three Doctors with the help of cue cards and pre-recorded inserts.