Fonseca is half-Portuguese. Born in Oakland, California, she was raised first in Alameda and then Moraga. She was discovered there by a manager and agent and immediately moved to Los Angeles for her first pilot season at age 13 in the middle of the school year. The following year she booked her breakthrough role, playing Colleen Carlton on The Young and the Restless with a three-year contract. With that, her parents moved the whole family to Los Angeles. Since the end of her contract, she has booked many television guest spots, television movies, two pilots and an independent feature film.
In 2001, Fonseca was signed by an agent and a manager and played troubled teen Colleen Carlton in CBS' 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 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).
In fall 2007, she played Dawn in the movie Remember The Daze. 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 and #89 in the 2011 list.
- Besides Maggie Q and Shane West, Lyndsy was in every episode of Nikita.
- Along with Simon Kassianides, she's was in a Marvel Cinematic Universe production. While Simon was in Marvel's Agents of S.H.I.E.L.D., Lyndsy was in Marvel's Agent Carter.