Eventually, Douglas's career took her in front of the camera with a small appearance in the 1973 film The Last Days of Man on Earth (alternatively known as The Final Programme). After this, Douglas became known to British television audiences in the 1970s by appearing in The Howerd Confessions, The Inheritors, Space: 1999 and Return of the Saint, and appeared in the films The People That Time Forgot (for which she was nominated for a Saturn Award), and the controversial and rarely seen film The Brute, in which she played an abused wife.
Douglas' first major role was that of Ursa in Superman (1978) and Superman II (1980) having beaten a staggering 600 actresses to the part. In 1984, she played another fantasy villainess in Conan the Destroyer as the evil Queen Taramis. Douglas also continued to appear on television, and was a series regular in the short-lived British sitcom Thundercloud in 1979. Throughout the 1980s, she went on to appear in a variety of guest roles in UK and US series such as The Professionals, Bergerac, , Hotel, Sledge Hammer!, and Remington Steele. However, her most prominent television role is that of Pamela Lynch in the primetime soap opera Falcon Crest, which she played for two seasons from 1983–85. She also played another "Pamela" in the 1984 television miniseries V: The Final Battle.
In the 1990s, she returned to science fiction, guest-starring in the Babylon 5 first season episode "Deathwalker" (1994), and in the Stargate SG-1 second season two-part episode "The Tok'ra". Douglas has also appeared in a variety of genre films, including Solarbabies, The Return of Swamp Thing, The Stepford Husbands, Beastmaster 2, and Return of the Living Dead 3. She also voiced characters in episodes of several animated TV series, including Iron Man, The Real Adventures of Jonny Quest, Superman: The Animated Series, and Batman of the Future.