She is best known for her role as Supervisory Special Agent Jennifer "JJ" Jareau on the CBS crime drama Criminal Minds (2005–2020, 2022–).

Cook has also appeared in The Virgin Suicides (1999), Higher Ground (2000), Ripper (2001), Out Cold (2001), Final Destination 2 (2003), and Tru Calling (2003–2004).

She spent most of her early life in nearby Whitby, where she attended Anderson Collegiate Vocational Institute.

Cook was declared legally blind in the second grade because of severe astigmatism, but wore contacts and thick "Coke bottle" glasses to correct her vision.

Talking about her childhood, Cook recalled how other kids were "cruel" and that she "really couldn't see" and "everything looked like a smear of colour with no shapes".

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.

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,[7] 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.

[11] Brewster also mentioned in a separate interview that someone from CBS called Bernero and said they wanted "new women" which led to their firing.