Underwood's film career began with roles in Just Cause (1995), Set It Off (1996) and Deep Impact (1998). He also had a supporting role as a geneticist in Gattaca. In 2000, he played the lead role in the short-lived television series City of Angels. In 2003, he guest starred in four episodes on the HBO series Sex and the City playing Cynthia Nixon's love interest. In 2004, he played the role of Roger De Souza opposite Heather Locklear in NBC's LAX. He gained acclaim as the sexy grade school teacher in the CBS sitcom The New Adventures of Old Christine opposite Julia Louis-Dreyfus for two years. In 2007, he guest starred in an episode of the NBC series Law & Order: Special Victims Unit. He played the character Alex in the first season of the HBO series In Treatment, for which he was nominated for best supporting actor at the 2009 Golden Globes.