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.
In 1959, she played a friend of Shelley Fabares on The Donna Reed Show, and had a supporting role in the sitcom The Betty Hutton Show, with her brother Gerald. In 1960, Perreau and Robert Harland performed as Sara Lou and Lin Proctor, a young couple from the east who have eloped and are heading west, in the western series Stagecoach West with Wayne Rogers and Robert Bray. Also in 1960, Perreau was cast as Julie Staunton in an episode of The Islanders, set in the South Pacific. She was cast in "Don Gringo" and "The Promise", as well as in The Rebel. In 1961, she played Mary Bettelheim in an episode of The Roaring 20s. She was cast in a recurring role on Follow the Sun series from 1961–1962 as secretary, Katherine Ann "Kathy" Richards. She guest starred on The Rifleman in 1960 and 1961. She made guest appearances on Perry Mason. In 1964, she also co-starred as Lucy, a beleaguered homesteader, on an episode of Gunsmoke. In 1970, she appeared on The Brady Bunch as a math teacher who becomes the object of puppy love by Greg Brady, one of her students.
In the 2000s, she provided her voice in the animated films Fly Me to the Moon, A Turtle's Tale: Sammy's Adventures and Crash: The Animated Movie, and acted in Time Again.