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 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.

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 voices of Lola Llama and Jungle Girl on two episodes of the Cartoon Network series My Gym Partner's a Monkey.