When she was seven, Cuthbert started modeling for various lines of children's clothing and also became a foot model. She made her first televised appearance as an extra in the horror-themed series for children Are You Afraid of the Dark?; she later became a series regular. Cuthbert also co-hosted Popular Mechanics for Kids, which was filmed in Montreal. Her reporting captured the attention of then-First Lady Hillary Clinton, who invited her to visit the White House.
Cuthbert landed her first role in a feature film in the family drama Dancing on the Moon (1997). She appeared in several other Canadian family-themed movies and an airplane thriller, Airspeed. Cuthbert starred in the Canadian television movie Lucky Girl in 2001 and was awarded a Gemini Award for her performance.
Soon after her move to Los Angeles, Cuthbert was cast in the role of Kim Bauer, daughter of CTU agent Jack Bauer in the television series 24. She appeared in the show's first three seasons, but not in its fourth; she guest-starred in only two episodes in the fifth season. She also reprised her role as Kim Bauer in 24: The Game. She has also said that she would like to play her role as Kim in the upcoming movie, 24: The Movie.
