Lea was born to mother, Edith Sarfati, a nurse, and father, Marc Sarfati, a delicatessen owner. She grew up an only child in Tenafly, New Jersey.
She attended Tenafly High School and Stagedoor Manor in the Catskills, a performing arts training center. She was then later accepted to the CAP21 program at the Tisch School of the Arts at New York University, but decided to continue working on stage instead. She made her Broadway debut in 1995 as a replacement cast member in Les Miserables as Young Cosette. Soon after, she moved to Toronto with her mother to act in the original company of Ragtime and in 1998 brought that show to Broadway.
In 2001, Lea went on her first audition for Spring Awakening, and she would be a part of all workshops for that show. In 2004 Lea portrayed Shprintze and Chava in the revival of Fiddler on the Roof and regionally played Anne Frank in The Diary of Anne Frank that same year. She was part of the original cast of Spring Awakening both off-Broadway and on Broadway as Wendla in 2006 through 2008. She was nominated for a Tony Award as Outstanding Performance by a Lead Actress in a Musical and a Drama Desk Award for Best Lead Actress. On May 18, 2008, Lea and costar Jonathan Groff had their final performances of Spring Awakening and she was replaced by Alexandra Socha, a former understudy.
On August 8-10, 2008 she played Eponine in Hollywood Bowl's concert of Les Miserables. She can be heard on the soundtracks for Ragtime, Spring Awakening, and Fiddler on the Roof. In May of 2009 after the finale of American Idol, Glee, a new comedy that features music on Fox, aired to much hype. Lea plays Rachel Berry on the show. The show returned in September 2009 for its initial 13 episode run, and has since been picked up for an additional back 9 for season one, as well as a 2nd season for the 2010-2011 Fox lineup.
She has won a Screen Actors Guild Award for outstanding ensemble performance and the 2009 Satellite Award for best actress. She also received nominations for an Emmy Award, two nominations for a Golden Globe Award, and Teen Choice Award for her performance as Rachel Berry. She is also an active member of PETA and various gay rights groups.