She remained with the show for all of the show's six seasons through 2009.In 2006, she starred in Brian De Palma's The Black Dahlia in which she plays the young aspiring actress Elizabeth Short, whose mutilation and murder in 1947 remains unsolved. It was supported logistically by Amnesty International, which will receive proceeds from the book.Kirshner also appeared in the first three episodes of 24 as the assassin Mandy in 2001.She would later reprise the role for the second season's finale and in the latter half of the show's fourth season.While the film itself was critically panned, many reviews singled out her performance for acclaim. In the book, four different groups of women and children refugees from places such as Chechnya, Juárez, Burma and Malawi tell their life stories. After the release of the book, the Center for International Studies at MIT invited Kirshner to run a 4-week course on I Live Here in January 2009.
Mia's first public relationship was with her own co-star from the show, , Jennifer Beals.
Also in 2001, Kirshner played Catherine Wyler, The Cruelest Girl in School, in Not Another Teen Movie.
The character is primarily a spoof of Kathryn Merteuil (played by Sarah Michelle Gellar) in Cruel Intentions, and was partially based on Mackenzie Siler (played by Anna Paquin) from She's All That.
As the series demanded a sex scene, Mia watched dozens of clips from movies like 'Bound, where the stars had an explicit affair.
In an interview to the New York Times, the actress said, She also got close with veteran actor Sam Shephard.