Caulfield's first notable role was as Brandon Walsh's girlfriend, Susan Keats, on Beverly Hills, 90210 in 1995. She appeared for thirty episodes in the series before departing in 1996. In 1998, Emma starred in her most famous role to date, as Anya Jenkins on the WB's hit show Buffy the Vampire Slayer. Originally, her character was only to appear for several episodes. However, audiences responded well to Anya, resulting in Buffy the Vampire Slayer creator Joss Whedon's decision to add her to the cast.
In 2003, Emma landed her first lead role in the horror movie Darkness Falls, which debuted at number one in the U.S. box office. In 2004, she appeared on Monk as Meredith Preminger in the episode "Mr. Monk and the Girl Who Cried Wolf". Caulfield also starred in the satire Bandwagon, playing a fictionalized and unflattering version of herself as a superficial, arrogant actress whose career is imploding. The movie was written and directed by close friend and fellow actress Karri Bowman. It screened at various festivals, although it has not been picked up for distribution. Several members of Buffy the Vampire Slayer cast and crew have cameos in the film.
She passed up the opportunity to audition for the roles of Six and Starbuck on Battlestar Galactica, a decision she later regretted