Cook's first job was in a
McDonald's commercial in 1997. She graduated to series work with a guest role on the television series
Goosebumps and parts in two television movies, ''In His Father's Shoes
and Elvis Meets Nixon''. Cook launched her feature film career as one of five suicidal Lisbon sisters in 1999's
The Virgin Suicides. Also that year, she took a starring role in the TV series
Higher Ground, in which Cook played Shelby Merrick, an adolescent girl dealing with abuse and heartache at a wilderness school for struggling teens.
Higher Ground lasted one season, ending with a romance between Shelby and
Hayden Christensen's character Scott. After
Higher Ground, Cook appeared in the 2000
TV movie The Spiral Staircase (alongside
Higher Ground alum
Kandyse McClure), and then captured leading roles in several films, including
Out Cold (as
Jason London's love interest),
Ripper, ''
I'm Reed Fish, and Final Destination 2. In 2003, Cook guest starred in Dead Like Me. She was also in Season One of Tru Calling'' as Lindsay Walker. Beginning in September 2005, she starred as
Jennifer "J.J." Jareau in the
CBS drama
Criminal Minds. On June 14, 2010, it was announced that her contract option would not be picked up for season six as a series regular reportedly due to budget cuts on the show, but she returned for two episodes to wrap up her character's storyline due to thousands of letters and petitions written to the show's producers. She also returned for one episode for the departure of
Paget Brewster from the series. Former executive producer of
Criminal Minds, Ed Bernero, later claimed that CBS executives called him one day and asked him to get rid of Cook and Brewster. Brewster also mentioned in a separate interview that someone from CBS called Bernero and said they wanted "new women" which led to their firing. She said that it was not due to budget cuts as their replacement was paid twice what they were paid. In April 2011, it was reported that Cook had signed to reappear for two more seasons of the show. In 2013, Cook and her
Criminal Minds co-star Kirsten Vangsness managed to negotiate salary raises but not parity after years of being paid less than half the salary of fellow co-stars Matthew Gray Gubler and Shemar Moore. In 2017, both women held out on signing new contracts as they sought pay parity with Gubler. They eventually succeeded and closed new deals to return. Cook would continue to star in the series until it ended in 2020 after 15 seasons. After the show wrapped, Cook decided to take a break from acting and in 2022, she guest-starred on the show
9-1-1. It was also announced that she would be returning for the
Criminal Minds revival. ==Other ventures==