Tilly, the third of four children, was born Margaret E. Chan in Long Beach, California, the daughter of Patricia Tilly, an Irish and Native American schoolteacher, and Harry Chan, a Chinese American used car salesman. Following her parents' divorce when she was three, she was raised by her mother and stepfather, John Ward, on rural Texada Island in British Columbia, Canada. Tilly graduated from Esquimalt High School in Victoria, British Columbia. She has a brother, Steve, and two sisters, Rebecca, and fellow actress Jennifer (born 1958).

As a teen, she was involved with the Connecticut Ballet Company, as well as the Throne Dance Theatre, and made her screen debut with Alan Parker's Fame in 1980.

After a serious back injury forced her to stop dancing, she became an actress. She played a prostitute in a second season episode of Hill Street Blues before she appeared in films such as The Big Chill and Psycho II before her acclaimed title role in Agnes of God , for which she received an Oscar nomination and won a Golden Globe. Tilly later appeared in Valmont , The Two Jakes with Jack Nicholson and Leaving Normal, as well as the 1993 horror film Body Snatchers .

Tilly was the first choice for the role of Constanze Mozart in Milos Forman's film Amadeus , having received glowing appraisals of her rehearsal work by both her would-be costar Tom Hulce and director Forman. However, she sustained a leg injury playing soccer and had to abandon the project. The role later went to Elizabeth Berridge .

Tilly portrayed the role of the Blessed Mother, a Pope-like figure in the Caprica episode " Unvanquished ." This was her first on-screen appearance since 1995.

Tilly has written a novel entitled Singing Songs , comprising a series of vignettes about a young girl whose stepfather molests her and her sisters. A second novel, Gemma , came out in October 2006. Her third novel, Porcupine , is a finalist for the Sheila A. Egoff Children's Literature Prize. Tilly also wrote a novel, First Time , published in 2008.

According to Tilly's book Singing Songs , she and her sisters grew up surrounded by poverty and abuse. They were sexually molested by their stepfather. The book is written in first person and the narrator has a very child-like voice. In her website foreword, Tilly states that although the stories are fictional, they are based on her childhood experiences.

Tilly has three children: Emily (born 1984) and David (born 1986), by her marriage to film producer Tim Zinnemann, which ended in 1989, and a son, Will (born 1990), from her five-year relationship with British actor Colin Firth. Her second husband, John Calley, was 30 years her senior. He is a former president of Sony Pictures; they were married in 1995 and divorced in 2002.

Now remarried and living in British Columbia, Tilly has not appeared in a film since 1995 (although she has made an appearance in Caprica in late 2010), having decided to retire from acting in order to raise her children and write.

Academy of Science Fiction, Fantasy, and Horror Films, USA

• 1984, Best Actress for Psycho II , (Nomination)

Academy Awards

• 1986, Best Actress in a Supporting Role for Agnes of God , (Nomination)

Golden Globe Awards

• 1986, Best Actress in a Supporting Role in a Movie for Agnes of God , (won)

Year	Title	Role	Notes
1980	''Fame (1980 film)|Fame''	Dancer		-	1981	''The Trouble with Grandpa''	Dorie	TV movie.
1982	''Hill Street Blues'' Ep: Some Like It Hot-Wired	Hooker		-	''Tex (film)|Tex''	Jamie Collins		-	1983	''One Dark Night''	Julie Wells		-	''Psycho II (film)|Psycho II''	Mary Loomis		-	''The Big Chill (film)|The Big Chill''	Chloe		-	1984	''Impulse (1984 film)|Impulse''	Jennifer		-	1985	''Agnes of God''	Sister Agnes		-	1986	''Off Beat (film)|Off Beat''	Rachel Wareham		-	1988	''Masquerade''	Olivia Lawrence		-	''The Girl in a Swing (film)|The Girl in a Swing''	Karin		-	1989	''Nightmare Classics'' Ep: Carmilla	Carmilla		-	''Valmont (film)|Valmont''	Madame de Tourvel		-	1990	''In the Best Interest of the Child''	Jennifer Colton	TV movie.
	''The Two Jakes''	Kitty Berman		-	1992	''Leaving Normal (film)|Leaving Normal''	Marianne			-	1993	''Road to Avonlea'' Ep: Evelyn	Evelyn Grier		-	''Body Snatchers (1993 film)|Body Snatchers''	Carol Malone		-	''Fallen Angels (TV series)|Fallen Angels'' Ep: Dead End for Delia	Lois Weldon		-	1994	''Winnetka Road'' TV series	George Grace		-	''Sleep with Me''	Sarah		-	''Trick of the Eye''	Faith Crowell	TV movies.
1995	''Journey''	Min
2010	''Caprica (TV series)|Caprica'': "List of Caprica episodes#ep10|Unvanquished"	List of Caprica characters#Mother|Mother