Trachtenberg made her first television appearance at the age of three, in a commercial for Wisk Detergent. She went on to feature in over 100 more commercials. She appeared in her first credited role as Nona F. Mecklenberg on the Nickelodeon series The Adventures of Pete & Pete from 1994 until 1996. During the same period, she played Lily Montgomery on the soap opera All My Children, in which she first worked with future Buffy the Vampire Slayer star Sarah Michelle Gellar.
Trachtenberg's film career began in 1996 with the titular role in Harriet the Spy, a film with screenplay written by Douglas Petrie. 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. She then starred in the short-lived television series Meego, which garnered her a Young Artist Award. She returned to film in 1999 for Inspector Gadget, as the inspector's niece, Penny. She also starred in the film Can't Be Heaven. In the summer of 2000, Trachtenberg took on the role of Dawn Summers in Buffy the Vampire Slayer; she remained in the role until the show ended in 2003. Trachtenberg also hosted the Discovery Kids series Truth or Scare, from 2001 to 2003.
After both series, she appeared in the comedy film EuroTrip. She also had a recurring role in the HBO series Six Feet Under. In March 2005, Trachtenberg played the title character in Walt Disney Pictures' family comedy-drama Ice Princess.
In April 2006, Trachtenberg guest starred in the episode of House, M.D., "Safe". In November 2006, she guest-starred in the sixth season of the Emmy-nominated crime drama Law & Order: Criminal Intent in the episode "Weeping Willow." Trachtenberg also made a cameo in the Fall Out Boy music video for "This Ain't a Scene, It's an Arms Race" (with fellow Buffy cast member Seth Green), Ringside's "Tired of Being Sorry", and Trapt's music video for "Echo".
In December 2006, Trachtenberg starred in Black Christmas. In 2007, she was cast as the female lead in an ABC comedy pilot called The Hill. Trachtenberg provided the voice of Tika Waylan for the direct-to-video animated movie Dragonlance: Dragons of Autumn Twilight.
Trachtenberg appeared on The CW hit show Gossip Girl as Georgina Sparks. In 2009, she was a cast member on the short-lived NBC drama Mercy. She returned to film with the 2009 picture 17 Again alongside Zac Efron, Sterling Knight and Matthew Perry. She also had a small role in the comedy film Cop Out. In March 2011, she was the featured cover girl in Maxim magazine.
On June 9, 2011, Trachtenberg guest-starred on Love Bites as Jodie. She reprised the role on June 16, 2011, then finally in the series finale on July 21, 2011. On June 28, 2011, it was announced Trachtenberg would guest star on Weeds during its seventh season, as the character Emma.
Trachtenberg appeared in all episodes from season 5 to season 7 of Buffy the Vampire Slayer, starring as Buffy's younger sister Dawn Summers:
- Buffy the Vampire Slayer season 5 (Guest Starring in "Buffy vs. Dracula")
- Buffy the Vampire Slayer season 6
- Buffy the Vampire Slayer season 7
- Michelle Trachtenberg on Wikipedia