Born Georgette Lizette Withers she began acting at the age of 12. A student at the Italia Conti Academy stage school, she was a dancer in a West End production when she was offered work as a film extra in Michael Powell's The Girl in the Crowd (1935). She arrived on the set to find one of the major players in the production had been dismissed, and she was immediately asked to step into the role.

During the 1930s she was constantly in demand in lead roles in minor films and supporting roles in more prestigious productions. Her best known work of the period was as one of Margaret Lockwood's friends in Alfred Hitchcock's The Lady Vanishes (1938).

Among her successes of the 1940s was the Powell and Pressburger film, One of our Aircraft is Missing (1942), a topical World War II drama in which she played a resistance fighter who helps British airmen return to safety from behind enemy lines.

Throughout her career she has appeared frequently in film, television, and theater. Married to Australian actor John McCallum since 1948, the couple met on a film set in Britain, and returned to Australia where both worked frequently in television together and appeared in a number of stage productions, also appearing together in the UK in Somerset Maugham's The Circle at Chichester Festival Theatre. They are the parents of the actors Nicholas McCallum and Joanna McCallum.

Withers' most recent screen performance was as the music teacher in the 1996 film Shine for which she and the other cast members were nominated for a Screen Actors Guild for "Outstanding Performance By A Cast".

Withers was made Commander of the Most Excellent Order of the British Empire (CBE) in 2002.