Fonseca was born in Oakland, California, the daughter of Lima Lynn (née Bergmann)[better source needed] and James Victor Fonseca. She is partly of Portuguese descent. Her mother later married attorney Reid Dworkin, through whom Fonseca has a half-sister, Hannah Leigh Dworkin. Fonseca graduated from Barbizon Modeling and Acting School in San Francisco and relocated to Los Angeles at age 12. She soon made her screen debut as Colleen Connelly-Carlton on the soap opera The Young and the Restless.
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).
In late 2007, Fonseca played Dawn in the film 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. In 2008 she was nominated for the Actor award at the Screen
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