Barton has since appeared in the comedy remake St Trinian's (2007), the Richard Attenborough-directed drama Closing the Ring (2007) and Assassination of a High School President (2008).
She returned to television, starring in the short-lived Ashton Kutcher-produced CW series, The Beautiful Life: TBL (2009).
The show became an overnight success due to its large fanbase, and resulted in Barton's fame being increased through her role as Marissa Cooper.
Barton departed from the show in the third-season finale in May 2006.
She is known for her role as Marissa Cooper in the Fox television series The O. (2003–07), for which she received two Teen Choice Awards.
The role catapulted Barton into mainstream fame, and Entertainment Weekly named her the "It Girl" of 2003.
In 2001, she returned to film and starred in the independent Canadian drama, Lost and Delirious, based on the novel The Wives of Bath by Susan Swan.The New York Times also remarked that "the winning child actresses Brittany Boyd and Mischa Barton are already smart enough not to overplay the naivete and precocity of their respective characters." The New York production went on to win the OBIE Award for Best Play.She made her film debut in Lawn Dogs, which won awards at film festivals around the world.The film was met with mixed reviews, but the performances of Barton and her co-stars Piper Perabo and Jessica Paré were widely praised.She also starred in Julie Johnson (2001) alongside Lili Taylor and Courtney Love, playing the daughter of a woman in a lesbian relationship.