Douglas started acting as a teenager, joining the Worthing, West Sussex repertory company, before making her West End theatre debut in 1958.
She made her (non-speaking) film debut in 1959 in The Shakedown, and then appeared with Tommy Steele in It's All Happening.
She is best remembered for her roles in several Carry On Films in the 1960s, including Carry On Cowboy (1965) as an all-singing and trigger-happy version of Annie Oakley. She then appeared in Carry On Screaming! (1966), Follow That Camel (1967) and Carry On Up the Khyber (1968). She has, by virtue of this association, appeared on many retrospective and spin-off programmes. Douglas made an appearance in North Wales in September 2005 to unveil a plaque dedicated to the filming of Carry On Up the Khyber, as part of the movie had been shot in Llanberis.
Her other films have included The Comedy Man (1964), Digby, the Biggest Dog in the World (1973) and The Four Feathers (2002).
Douglas met fellow actor Kenneth More on the set of Some People in Bristol in 1962. After starting an affair, More divorced his wife, and the couple married on 17 March 1968. More died on 12 July 1982.
After husband Kenneth More was diagnosed with Parkinson's Disease, she put her career on hold, after 11 episodes of Oil Strike North. After More's death, she returned in various roles, including Doris Lethbridge-Stewart in Doctor Who (1989) and Peak Practice. In 1996, she played the part of Isobel Trimble in the third series of Cardiac Arrest. She has since concentrated on a career in journalism and writing, having completed two books, including the autobiographical Swings and Roundabouts.