He began acting in the mid-1990s, and appeared in several made-for-television films and series, including Buffy the Vampire Slayer and Mad About You. He had supporting roles in several feature films, including Jack Frost and Hearts in Atlantis. In 1999, he received The Hollywood Reporter "Young Star Award" for his portrayal of John in David Mamet's The Cryptogram at the Geffen Playhouse. Rothhaar's biggest role to date was the leading role in Kart Racer (2003). In 2004, he had a starring role on Listen Up!. He appeared in Must Love Dogs (2005) and as Cpl. Lee Imlay in the alien-invasion film Battle: Los Angeles (2011). He appeared on two episodes of CSI: Crime Scene Investigation (in 2004 and 2010, respectively), CSI: Miami (2011), and CSI: New York (2012), playing different characters each time. In 2013, he starred as Lee Harvey Oswald in the television film Killing Kennedy.