In 2001, Fonseca was signed by an agent and a manager and played troubled teen Colleen Carlton in CBS' number one daytime drama The Young and the Restless. She also has recurring roles as Ted Mosby's future daughter on How I Met Your Mother and the Donna on HBO's Big Love.
In fall 2007, she played Dawn in the movie Remember The Daze, a remake of the 1999 graduating class. She joined the cast of ABC's Desperate Housewives as the daughter of Katherine Mayfair (Dana Delany), a character moving onto Wisteria Lane. Fonseca also appeared in an episode of Switched.
In 2008 she was nominated for the Actor award at the Screen Actors Guild Awards for Outstanding Performance by an Ensemble in a Comedy Series for Desperate Housewives (2004) shared with the cast.
Fonseca played Katie Deauxma in the 2010 superhero film Kick-Ass. She appeared in three more US films in 2010, including Hot Tub Time Machine.
In 2010, she was cast in the CW's Nikita as Alex, a new recruit.
She was listed at #62 in the 2010 Maxim Hot 100.
Fonseca has been sent a script for the adaption of Suzanne Collins's young adult novel The Hunger Games and will possibly audition for the role of its protagonist, Katniss Everdeen.
