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. At two years old, she was reading college textbooks. Witt was discovered by David Lynch in 1980 when she recited Shakespeare's Romeo and Juliet on the television show That's Incredible!. He cast her in the movie Dune (1984) as Paul Atreides's sister Alia. She turned eight during filming. She worked with Lynch again when she appeared in an episode of Twin Peaks, playing the younger sister of Lara Flynn Boyle's character Donna.
At age 14, Witt earned her high school diploma. Shortly thereafter, she moved to Hollywood with her mother to pursue acting. In September 1990, she competed on Wheel of Fortune.
During this time, Witt supported herself by playing piano at the Regent Beverly Wilshire Hotel. She had small parts in Mike Figgis's Liebestraum (in which her brother Ian also appears), the Gen-X drama Bodies, Rest & Motion and the TV movie The Disappearance of Vonnie. In 1994, Witt landed her first lead role in a film, playing a disturbed teenager in Fun. She received the Special Jury Recognition Award at the Sundance Festival and was nominated for Best Actress at the Independent Spirit Awards. This performance inspired Madonna to recommend Witt be cast as her daughter in the first segment of Four Rooms ("The Missing Ingredient").
Witt was introduced to a larger audience in the role of Zoey Woodbine, daughter of actress Cybill Shepherd's character in the sitcom Cybill. While playing that part from 1995 to 1998, she had film roles in Mr. Holland's Opus, Alexander Payne's abortion comedy Citizen Ruth, Passion's Way and Bongwater. After Cybill was cancelled, Witt had leading roles in the horror film Urban Legend and the animated feature Gen¹³, which was never released because the studio withdrew funding before its completion.
In 2000, Witt had starring roles on the television shows Ally McBeal and The Sopranos; the lead role in the comedy Playing Mona Lisa; a supporting part in John Waters's Cecil B. Demented; and her stage debut at Los Angeles's now-closed Tiffany Theater in Robbie Fox's musical The Gift, in which she played a high-priced stripper with a disease.
In the following years, Witt's acting career slowed. She had a small part in Cameron Crowe's Vanilla Sky intended as a reference to her roles in Dune and Liebestraum; played a college graduate who discusses losing her virginity in the experimental Ten Tiny Love Stories; and was the trailer-trash Barbie in American Girl, which was released to video in 2005.
Witt appeared in the 2002 romantic comedy Two Weeks Notice starring Hugh Grant and Sandra Bullock. In 2003-04, she lived primarily in the United Kingdom, filming The Upside of Anger opposite Kevin Costner and starring as Evelyn in a stage production of Neil LaBute's The Shape of Things. Between these two projects, she went to South Africa to shoot the German TV movie Kingdom in Twilight, which also goes by the names The Sword of Xanten, The Ring of the Nibelungs and, in the United States, Dark Kingdom: The Dragon King. Witt played Kriemhild in this film interpretation of the epic poem Das Nibelungenlied. On June 14, 2004, Witt modeled what is believed to be the most expensive hat ever made for Christie's auction house in London. The Chapeau d'Amour, designed by Louis Mariette, is valued at $2.7 million (US) and is encrusted in diamonds. In September 2006 she returned to the London stage at the Royal Court Theatre in Piano/Forte, portraying the piano-playing Abigail, a role in which she demonstrated herself to be an "outstanding pianist" of "formidable skill".
Witt joined the cast of Law & Order: Criminal Intent for the 2007-08 season as Detective Nola Falacci, a character who was a temporary replacement for Megan Wheeler (played by Julianne Nicholson, who was away on maternity leave). She was a recurring character in Friday Night Lights for the 2009-10 season. In May 2011, Witt was in talks with Law & Order: Special Victims Unit producers to reprise her Criminal Intent role if female lead Mariska Hargitay left the series, but Hargitay chose to stay. In March 2012, Witt retweeted a link on her official Twitter page in hopes of getting Law & Order: Criminal Intent back for an 11th season on NBC.