Mara made her film debut in Random Hearts, with Harrison Ford in 1999, directed by Sydney Pollack.
She played Jessica Chandler, the daughter of a congresswoman (Kristin Scott Thomas).
She had a recurring role on the WB's Jack & Bobby in 2005 and a five-episode arc on the Fox TV series 24 in 2006, playing computer analyst Shari Rothenberg.
Mara joined the cast of the HBO comedy-drama Entourage for the series' sixth season in 2009.
Mara landed two recurring roles in 2003, as Kate on Everwood, an 18-year-old who is impregnated by her piano teacher and then gets an abortion, and as Vanessa on Nip/Tuck, a bisexual cheerleader involved in a love triangle with her boyfriend Matt Mc Namara (John Hensley) and another cheerleader (Sophia Bush).
Mara began acting at the age of nine in a school musical.
She attended several youth theater-arts schools and appeared in community theater and in school plays.
Her first audition was for the NBC police drama Homicide: Life on the Street.
She did not get the role but knew from then on that she just wanted to act.