Michelle Trachtenberg, a touchstone of millennial youth culture who grew up onscreen, rising to fame as a troubled teenager on the supernatural 1990s series “Buffy the Vampire Slayer” and as a conniving young socialite on “Gossip Girl,” was found dead on Wednesday in Manhattan. She was 39.

The New York Police Department said in a statement that officers, responding to a 911 call just after 8 a.m. on Wednesday, found Ms. Trachtenberg unconscious and unresponsive in a Manhattan apartment. She was pronounced dead by emergency medical workers, who had also responded.

The department said that the medical examiner would determine the cause of death, but added that criminal activity was not suspected.

Ms. Trachtenberg established her longstanding presence on the small screen at the age of 3, when she appeared in a television commercial for Wisk laundry detergent in which she spilled cranberry juice.

Before she was 10, she was making regular appearances on the Nickelodeon sitcom “The Adventures of Pete & Pete,” as well appearing on the enduring ABC daytime drama “All My Children.”