Tunney was born in Chicago, Illinois on June 19, 1972 and raised on the South Side of Chicago. During her freshman year of high school, she attended St. Ignatius College Prep, a co-educational Catholic prep school in Chicago. She then went on to study acting at the Chicago Academy for the Performing Arts. At the age of 18, Tunney moved to Los Angeles, California and obtained several recurring TV roles on Class of '96, Law & Order, Dream On, and Life Goes On.
Her major breakthrough role was as a suicidal teenager in Empire Records before receiving the leading role as a gothic witch in The Craft, alongside Fairuza Balk, Neve Campbell and Rachel True. Robin cut her hair for her role in Empire Records, and had to wear a wig in The Craft.
In 1997, Tunney starred in Bob Gosse's Niagara, Niagara with Henry Thomas in a performance which garnered her the Best Actress award Volpi Cup at the 1997 Venice International Film Festival. Tunney also starred opposite Arnold Schwarzenegger in the 1999 action film End of Days.
She was ranked #81 on the Maxim Hot 100 Women of 2001
She also stared in the House episode 'Pilot' as a kindergarten teacher who collapses before her class and descends into dysphasia. She played in Castle a police agent.
In 1997, Tunney married producer/director Bob Gosse. They were separated in 2002.
On June 28, 2006 Tunney won the 5th table of Bravo's Celebrity Poker Showdown's 8th season, finishing ahead of Christopher Meloni, Macy Gray, Joy Behar, and Andy Dick, moving on to the final table. The finale aired on July 5, 2006, where she competed against Jason Alexander, Michael Ian Black, Ida Siconolfi, and Keegan-Michael Key. Tunney came in second, behind Alexander, and ahead of Black, Siconolfi, and Key. She played for The Children's Health Fund, which received $200,000 for her efforts.
In August 2006 Tunney played in the WSOP after having her entrance fee covered by the online cardroom PokerRoom.com