Witt was born in Worcester, Massachusetts, the daughter of Diane, a junior high school reading teacher, and Robert Witt, a science teacher and photographer. She has a brother, Ian. Witt was discovered by David Lynch when she appeared on the television show That's Incredible! in 1980; she had recited Shakespeare's Romeo and Juliet. He cast her in the movie Dune (1984), where she played Paul Atreides' young sister Alia. She turned eight during filming. Afterwards, she left Hollywood to concentrate on her studies and music. She was home schooled by her parents. She won several national and international classical piano competitions, including the "Bartok-Kabalevsky International Piano Competition".

At age 14, Witt earned her high school diploma. Shortly thereafter, she moved to Hollywood with her mother (who was noted from 1988-93 in the Guinness Book of Records for the world's longest hair; it is not known for sure if she still has her long hair) to pursue a career as a full-tim