Daughter of French-born Robert Perreau-Saussine and Eleanor Child Perreau-Saussine, she was born Ghislaine Elizabeth Marie Thérèse Perreau-Saussine. Perreau achieved success as a child actress in a number of films. She got into the business quite by accident. Her older brother Gerald was trying out for the part of the title character's son in Madame Curie. Because their mother could not find a babysitter, she took Gigi along. The two-year-old, who could speak French, got the (uncredited) part of Madame Curie's daughter Ève (while Gerald would have to wait a year to make his film debut in Passage to Marseille).

She also played the daughter of Claude Rains and Bette Davis's characters in the 1944 film Mr. Skeffington. In Shadow on the Wall, she starred as the sole witness to a murder. As the "top child movie actress for 1951", the then ten-year-old was given the keys to the city of Pittsburgh by its mayor, and later Pennsylvania governor, David L. Lawrence. She was the youngest person to be so honored. Perreau played the rebellious teen daughter of Fredric March in 1956's The Man in the Gray Flannel Suit. However, her film career lost momentum as she became an adult, so she turned to television.