She began her career by appearing as Colleen Carlton on the CBS daytime soap opera The Young and the Restless, on which she starred between 2001 and 2005.

From 2010 to 2013, Fonseca starred as Alex Udinov on The CW's Nikita, and from 2015 to 2016 she played Angie Martinelli on ABC's Agent Carter.

She has also appeared in a variety of film roles, including Jenny in Hot Tub Time Machine (2010) and Katie Deauxma in Kick-Ass (2010) and its 2013 sequel.

Fonseca completed a course at Barbizon Modeling and Acting School in San Francisco[7] and moved to Los Angeles at the age of 12.

[1] In 2005, Fonseca played a recurring role as Ted Mosby's future daughter on How I Met Your Mother and as Donna on HBO's Big Love.

She starred in the 2005 Hallmark TV film Ordinary Miracles, as a 16-year-old juvenile delinquent who goes to live with a judge (Jaclyn Smith).