Witt was born in Worcester, Massachusetts. Her mother, Diane (née Pietro), is a junior high school reading teacher, and her father, Robert Witt, is 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".