In 2007, she was cast as the female lead in an ABC comedy pilot called The Hill, based on the newspaper of the same name in Washington, D. In 2009, Trachtenberg was a cast member on the NBC drama Mercy; the series lasted one season.
In 2006, Trachtenberg starred in Black Christmas, the remake of the 1974 slasher of the same name.She reprised the role on June 16, 2011, and in the series finale on July 21, 2011.In the same year, Trachtenberg was a guest-star on Weeds during its seventh season, In the Criminal Minds episode "Zugzwang", Trachtenberg played Diane Turner, the criminal stalker to Spencer Reid's girlfriend, Maeve Donovan. She also had a small role in the comedy film Cop Out.During this time, she also began a recurring role in season two of the television series The Adventures of Pete & Pete, of which she later became a regular. In the summer of 2000, she took on the role of Dawn Summers in Buffy the Vampire Slayer, remaining in the role until the show ended in 2003.She then starred in the short-lived television series Meego, which garnered her a Young Artist Award. She also hosted the Discovery Kids series Truth or Scare from 2001 to 2003.