Tohn was born and raised in Oceanside, Long Island, New York, to physical education teachers Alan and Bella Tohn. She has two older brothers. Tohn began acting professionally at age 10, with an extra role on the PBS children's series Ghostwriter, and was a student at Boardman Middle School when she had her first major role, an appearance on the CBS sitcom The Nanny. After high school, she went to the University of Delaware and majored in elementary education, and in the fall of 1998 was in the a cappella group the Deltones. In November 1999, she starred in the one-act play "This Property is Condemned" by Tennessee Williams at the Chapel Street Theatre.