Quinlan was born in Pasadena, California, the daughter of Josephine (née Zachry), a military supply supervisor, and Robert Quinlan, a television sports director.

She was raised in Mill Valley, California, where she attended Tamalpais High School and began her acting career. She has been married to actor Bruce Abbott since April 12, 1994. They have two children.

Quinlan made her film debut in George Lucas' 1973 nostalgic look at the early sixties, American Graffiti at the age of 19 (although she did appear in an uncredited role in 1972's One is a Lonely Number).

She has appeared in over 50 films, but is best known for her roles as Deborah, a schizophrenic, in the film version of I Never Promised You a Rose Garden for which she earned a Golden Globe nomination for Best Actress-Motion Picture-Drama, and for her Golden Globe and Oscar nominated role for Best Supporting Actress as Marilyn Lovell in 1995's Apollo 13.

She also made a mark as Jim Morrison's Celtic Pagan lover Patricia Kennealy in Oliver Stone's The Doors, and won a Blockbuster Entertainment Award as Favorite Supporting Actress-Suspense, for Breakdown (1997).

Her most recent work includes the TV series House, the 2006 remake of the horror classic "The Hills Have Eyes", and in the 2007 film, Breach.