She also starred in the critically acclaimed indie crime drama Pups (1999).
Barton later appeared in the independent drama, Lost and Delirious (2001) and guest-starred as Evan Rachel Wood's girlfriend on ABC's Once and Again (2001–02).
She made her screen debut with a guest appearance on the American soap opera All My Children (1996).
In 1996, she returned to the show, replacing Michelle Trachtenberg in the role of Lily Benton Montgomery.She guest-starred in eight episodes of the television series Once and Again as Jessie Sammler's (Evan Rachel Wood) girlfriend, Katie Singer.Barton began acting at the age of eight, co-starring in the Off-Broadway premiere of the 1994 play, Slavs! Vincent Canby of The New York Times praised Barton's "so fine" and "chillingly authoritative" performance.New York magazine also praised her as "a darling little girl, [that] exhibits consummate charm".She made her film debut in Lawn Dogs, which won awards at film festivals around the world.
Barton won critical favour for her role, with Janet Maslin of The New York Times praising her "poised" performance Barton then appeared in Notting Hill (1999) and The Sixth Sense (1999).star."It's early days yet but, yes, we are seeing each other. Played socialite Marissa Cooper on the television series The O. and also appeared in the suspense film The Sixth Sense. She appeared in several other Off-Broadway productions, taking a lead role in James Lapine's Twelve Dreams alongside Marisa Tomei at Lincoln Center.Canby again praised Barton, noting that she "has a sweet gravity as the doomed Emma".She also starred in Julie Johnson (2001) alongside Lili Taylor and Courtney Love, playing the daughter of a woman in a lesbian relationship.