Stewart was born on 22 April 1959, in Edmonton, Alberta, the daughter of Mary (Stewart) and John Ralph Nursall. Her parents taught at the University of Alberta, her mother a physiology teaching assistant and her father a biology professor. Stewart attended Strathcona Composite High School.

She was married to actor John Findlater in 1983 and divorced in 1985. She married Richard Allerton in 1992. She has a daughter and a son and lived in Brooklyn, New York, as of 2013.

After moving to Los Angeles she obtained a role on the soap opera Days of Our Lives, playing the original Kayla Brady from January 1982 to December 1983. In 1984, she starred in two science fiction feature films, The Last Starfighter as Maggie Gordon and Night of the Comet as Regina Belmont. She later played a leading role in the teen comedy Mischief.

In the mid-1980s, Stewart appeared in two miniseries: Hollywood Wives (1985) and Sins (1986), where she played the younger version of Joan Collins’s character. She made guest appearances on television series such as Knight Rider, Hotel, Alfred Hitchcock Presents and The Outer Limits. She starred in several made-for-TV movies such as Murder by the Book (1987), Passion and Paradise (1989), Perfect Harmony (1991) and Ordeal in the Arctic (1993). In 1989, she appeared as Gwen Saunders in the comedy film Weekend at Bernie’s.