Moreau's first break out role was playing the recurring part of Melanie on Blossom in 1994 and 1995. She guest starred in several TV series such as Boy Meets World and 3rd Rock from the Sun. As a child actress, she appeared in the Mighty Ducks films playing Connie Moreau. After graduating from Vassar College with her B.A. in Political Science in 1999, she landed some of her most prominent roles. After starring in the cult film Wet Hot American Summer, she played the role of Jesse in the 2002 film Queen of the Damned, based on the Anne Rice novels.

She played the lead role in the TV miniseries Firestarter 2: Rekindled, succeeding Drew Barrymore in the role of Charlie McGee. In 2003's Runaway Jury she played opposite Gene Hackman. Moreau also starred in the 2003 independent film Easy, which has also been broadcast on the Showtime cable television network. She stars as Jamie, a career oriented woman who can't seem to find a nice guy. Then she meets two nice guys, John (played by Naveen Andrews of Lost) and Mick (played by Brian F. O'Byrne), and is unable to decide between the two of them.

Returning to television, she starred as Susan Atkins in Helter Skelter, a television movie about Charles Manson, and had a regular role on the ABC network series Life As We Know It in the 2004–2005 season. She played Monica Young, a high school teacher having an affair with one of her students. Moreau appeared in a season two episode of Lost, playing Starla in "Everybody Hates Hugo", as well as several episodes of The O.C., playing Reed Carlson. Originally, she was to have a regular role in the Fox show Killer Instinct, but her character was written out of the series after the first episode and was replaced by Kristin Lehman. She has also appeared as a recurring character on the ABC series What About Brian. She also appeared in a season 2 episode of Mad Men as an enigmatic call girl named Vicky.

• Easier with Practice (2009) (post-production) .... Samantha
  
• Wake (2009) .... Lila
• Lightbulb (2009) .... Cinda
• "Life" .... Betsy Bournes (1 episode, 2008)
       - The Business of Miracles (2008) TV episode .... Betsy Bournes
• Beverly Hills Chihuahua (2008) .... Blair
• "Mad Men" .... Vicky (1 episode, 2008)
       - Three Sundays (2008) TV episode .... Vicky
• The Consultants (2008) (TV) .... June
• The Uninvited (2008) .... Lee
• "Ghost Whisperer" .... Lisa Bristow (1 episode, 2007)
       - Dead to Rights (2007) TV episode .... Lisa Bristow
• "What About Brian" .... Suzanne (6 episodes, 2006)
       - What About the Fish (2006) TV episode .... Suzanne
       - What About Denial... (2006) TV episode .... Suzanne
       - What About the Wedding (2006) TV episode .... Suzanne
       - Sex, Lies and Videotape (2006) TV episode .... Suzanne
       - The Importance of Being Brian (2006) TV episode .... Suzanne
        (1 more)
• "Lost" .... Starla (1 episode, 2005)
       - Everybody Hates Hugo (2005) TV episode .... Starla
• "Killer Instinct" .... Detective Ava Lyford (1 episode, 2005)
       - Pilot (2005) TV episode .... Detective Ava Lyford
• "The O.C." .... Reed Carlson (4 episodes, 2005)
       - The O.Sea (2005) TV episode .... Reed Carlson
       - The Showdown (2005) TV episode .... Reed Carlson
       - The O.C. Confidential (2005) TV episode .... Reed Carlson
       - The Rager (2005) TV episode .... Reed Carlson
• "Life As We Know It" .... Monica Young (10 episodes, 2004-2005)
       - Breaking Away (2005) TV episode .... Monica Young
       - A Little Problem (2005) TV episode .... Monica Young
       - Family Hard-ships (2004) TV episode .... Monica Young
       - With a Kiss, I Die (2004) TV episode .... Monica Young
       - Natural Disasters (2004) TV episode .... Monica Young
        (5 more)
• Sucker Free City (2004) (TV) .... Jessica Epstein