Hagan's first major break came when she was cast in a recurring role as Millie Kentner on the short-lived NBC cult TV series Freaks and Geeks (1999-2000). Following that show's cancellation, she was cast on the David Alan Grier NBC sitcom DAG, where she was originally slated to play Camilla Whitman, daughter of the US President; however, the role of Camilla was recast when the pilot episode was revamped before airing.
Following the lead of several fellow Freaks and Geeks cast members, Hagan then appeared in an episode of the short-lived Fox sitcom Undeclared in 2001. Her next roles were a bit part in the Jack Black movie Orange County and a guest role as Melissa in Boston Public in 2002. After that, she was cast as Amanda in the seventh and final season of the hit cult series Buffy the Vampire Slayer.
In recent TV roles, Hagan appeared on the ABC medical drama Grey's Anatomy as Devo, a teenage girl whose religious beliefs conflict with getting treatment for a heart condition she was diagnosed with; she appeared on the NBC drama Medium, playing a character named Suzannah, a friend of Allison's (Patricia Arquette) who appeared in two dreams flashing back to Allison's teen years; and she played the role of Patricia on an episode of the CBS legal drama Close to Home. She made her first venture into animation in 2007 when she played the voice of Lola Llama on an episode of the Cartoon Network series My Gym Partner's a Monkey.
Hagan will appear next in the upcoming Warner Bros. comedy feature Spring Breakdown (which is scheduled for a DVD release on October 25, 2008), alongside Saturday Night Live cast members Amy Poehler and Rachel Dratch; she will play Truvy, the friend of a college co-ed (Amber Tamblyn) who is also the daughter of a powerful senator. She will also portray Natalie in the upcoming comedy film Thunder Geniuses.