Following her parents' divorce when she was three, Tilly was raised by her mother and stepfather, John Ward, on rural Texada Island in British Columbia, Canada.

[3] Forced to give up dancing because of complications stemming from her back injury, Tilly moved to Los Angeles to pursue a career as an actress and studied acting under Peggy Feury.

She made her television debut in the 1982 half-hour drama The Trouble with Grandpa, co-starring Elisha Cook Jr. After playing a prostitute in a second-season episode of Hill Street Blues, she appeared in her first starring role in the 1982 coming-of-age adventure film Tex with Matt Dillon.

In 1983, after she starred as the lead in the supernatural horror film One Dark Night, she appeared in Psycho II with Anthony Perkins, and Lawrence Kasdan's award-winning ensemble film The Big Chill, with Kevin Kline, Glenn Close, Tom Berenger, William Hurt, Jeff Goldblum, JoBeth Williams and Mary Kay Place.

In 1985, Tilly landed the acclaimed title role in Norman Jewison's Agnes of God, appearing with Jane Fonda and Anne Bancroft.

Playing the role of a novitiate nun who confesses her involvement in a virgin conception, Tilly "delivered a magnificent portrayal of a tormented young woman experiencing the ultimate crisis of faith".

[7] Tilly later appeared in Valmont (1989), The Two Jakes (1990) with Jack Nicholson and Leaving Normal (1992) with Christine Lahti, as well as the 1993 horror film Body Snatchers.

[10] In January 2012, Global in Canada launched the six-part Bomb Girls about women who work in a munitions factory during World War II.

[16] The book is about a twelve-year-old girl, Jacqueline "Jack" Cooper, whose life is shattered by the death of her father by friendly fire in the War in Afghanistan.

[18] A departure from the darker themes of Tilly's previous work, the novel is about two young girls who become friends who experience the "comical, sometimes bittersweet and melodramatic trials and tribulations of tweenhood".

[19] One reviewer wrote, "Tilly paints an insightful, memorable portrait of the ups and downs of friendship and the unwavering bonds of family, delving into age-old issues of honesty, trust, and loyalty.

[20][21] In 2018, Tilly published the first of three books in her Solace Island trilogy, a series of romantic thrillers about a young woman, Maggie Harris, dating a mysterious handsome man after recently being dumped by her fiancé.

They moved from Los Angeles to a log house on five acres of mountainside property about an hour outside Vancouver near the town of Maple Ridge, British Columbia.