Born on the south side of Chicago, Illinois, Tunney attended a Catholic High School at Saint Ignatius College Preparatory.

She went on to study acting at the Chicago Academy for the Performing Arts, spending her summers performing in such plays as Bus Stop and Agnes of God.

At the age of eighteen, Tunney moved to Los Angeles, where she began finding work on various television shows like Law & Order, Life Goes On, Dream On and a recurring role on Class of '69.

The actress made her screen debut in the 1992 Brendan Frasercomedy Encino Man (1992). She had her first lead role in the teen ensemble film Empire Records (1995), playing a suicidal record store employee who announces her presence in the film by walking into the store and shaving her head.

Her first starring role came in 1996's The Craft, opposite Neve Campbelland Fairuza Balk. Focusing on a group of misfit teens, Tunney played Sarah, the vulnerable newcomer to the coven of teenage witches. Since this film, she has landed a great number of starring and co-starring roles in films like Julian Po (1997), End of Days (1999), Supernova (2000) and Vertical Limit (2000). She also found work in independent films such as Niagara Niagara which earned her the Venice Film Festival's Volpi Cup for Best Actress. In addition, Tunney continues to make appearances on television, especially telefilms like Rescuers: Story of Courage: Two Families and Naked City: Justice with a Bullet.