Barton was born in Hammersmith, London, to an Irish mother, Nuala (nee Quinn), a photographer, and an English father, Paul Marsden Barton, a foreign exchange broker from Manchester. Barton's grandfather on her mother's side was an Irish language professor at the University of Belfast. She has two sisters, younger Hania Barton and elder Zoe, a barrister in London. Barton has stated that she briefly attended St. Paul's Girls' School in Hammersmith, but her father's work took the family to New York City when Barton was five years old. In 2006, ten days after her twentieth birthday, she became a naturalised citizen of the United States. She has retained her British citizenship. She has stated, "I like people to discern that I'm an English-Irish girl who grew up in New York City and who moved around a lot with her dad's job."
Barton graduated from the Professional Children's School in Manhattan in 2004, and took a summer short course called Acting Shakespeare at The Royal Academy of Dramatic Art in London, in June and July 2006, at Sir Richard Attenborough's urging, after he directed her in Closing the Ring.
Barton began acting at the age of eight, co-starring in the Off-Broadway premiere of the 1994 play, Slavs!, written by Tony Kushner. 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 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". In 1996 Barton had a supporting role in Catherine Butterfield's Where the Truth Lies which opened at New York's Irish Repertory Theatre. 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." In 1997, she appeared alongside Diane Wiest at The Public Theater in Naomi Wallace's One Flea Spare. 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. Barton won critical favour for her role, with Janet Maslin of The New York Times praising her "poised" performance and Empire celebrating her "hypnotic central performance". Barton then appeared in Notting Hill (1999) and The Sixth Sense (1999). Both films were critically and commercially successful, respectively earning $363,889,700 and $672,806,292. She also starred in Pups, a "Bonnie and Clyde for the MTV generation", alongside Burt Reynolds; along with her co-star Cameron Van Hoy, film critic Roger Ebert called their work in the film "two of the most natural and freed performances I have seen by actors of any age."
Barton first appeared on television in 1994, playing a young Corvina Lang in flashbacks in the soap opera All My Children. 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. 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 film was met with mixed reviews, but the performances of Barton and her co-stars Piper Perabo and Jessica Pare 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. Barton also had a supporting role in the independent teen drama Tart (2001) with Brad Renfro and Dominique Swain.
Other television appearances include an episode of the Fox series Fastlane (2003). She also appears as the love interest in James Blunt's music video "Goodbye My Lover" (2005) and in Enrique Iglesias' music video "Addicted" (2003).
In August 2003, Fox premiered the television series The O.C., about affluent teenagers with stormy personal lives who reside in scenic Orange County, California. 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. During her time on the show, Barton garnered several Teen Choice Awards. She was also celebrated by Glamour as the "Next Big Thing" and as Entertainment Weekly's "It Girl 2003" in the magazine's "It List" for top Creative People in Entertainment. The O.C. dropped in ratings dramatically during its third and fourth seasons, and ended in early 2007.
In July 2006, Barton appeared on the British comedy variety show The Friday Night Project as a guest host. In 2007, Barton appeared in the films The Oh in Ohio and the comedy film Virgin Territory with Hayden Christensen. She also joined Shirley MacLaine and Christopher Plummer in Richard Attenborough's war drama Closing the Ring. Attenborough praised her, saying "her ability to convey pain and sadness, hopefulness and fear, is riveting". The Independent also noted that "she lends her character a beguiling vulnerability."
She also appeared in the British comedy St Trinians (2007), appearing as a former head girl of that school advising current senior pupils on fashion and style. Later in 2007, Barton was cast in the international film produced by the Russian band t.A.T.u., called You and I. The film premiered at the Cannes Film Festival in May 2008 and was released theatrically in Russia in 2011. She appeared in Assassination of a High School President (2008), co-starring Bruce Willis, which premiered at the 2008 Sundance Film Festival. The Hollywood Reporter praised Barton's "captivating" performance.
She also starred in the horror film Walled In, which was released straight-to-DVD in March 2009. It received mixed reviews, but her performance was praised, with IGN calling her performance "terrific". Her next horror project, Homecoming, received a limited release in July 2009. Barton did not attend the premiere of the psychological thriller due to a medical issue. She also starred in the 2009 film Don't Fade Away. In early 2009 she began filming alongside Martin Sheen in Bhopal: A Prayer for Rain a biopic of the Bhopal gas tragedy. The title is currently in post-production and slated for a 2013 release.
In a December 2007 interview, she indicated that she would return to television for the right project; "It's not that I dislike doing television at all. It would be interesting having more creative control over where the storylines go or the characters. But for the right thing, of course." In 2009 she starred as one of the main characters on The Beautiful Life a new series on The CW. The series was subsequently cancelled on 25 September 2009 after televising 2 episodes. In December 2009, the technology company HP became the show's sponsor and began airing the show's five episodes on YouTube.
She guest starred on a Law & Order: Special Victims Unit, played an endangered prostitute in an episode that aired on 3 March 2010. She has since mainly filmed horror and thriller projects, taking roles in The Sibling, Cyberstalker, a Lifetime television film and Apartment 1303 3D, the English-language remake of the Japanese horror film, Apartment 1303. Barton's most recent film appearance was as the protagonist in Mark Edwin Robinson's supernatural romance thriller, Into the Dark.
In May 2012, Barton discussed her recent preference for the horror genre, following roles in Homecoming, Apartment 1303 and Sibling "I literally became obsessed with that genre. I can't tell you. Because I never watched it when I was younger; it used to be way too terrifying for me." However Barton added that, "I think I've gone through my little phase, for the moment." In June, Barton appeared in the new music video of Noel Gallagher's Everybody's on the Run.
In July 2012, it was revealed that Barton would make her stage return in an Irish production of Steel Magnolias. Barton will play Shelby in the production, which will premiere at the Gaiety Theatre, Dublin in September and will be followed by a nationwide tour. After the premiere on 12 September, Barton received praise for her performance. The Evening Herald wrote "Barton sparkles from the moment she steps on stage...Barton's Shelby is instantly magnetic".
She is scheduled to begin filming the independent drama, Fast Life in Los Angeles in summer 2013.
Barton has been in many television commercials and print advertising campaigns. Barton has modelled for companies Calvin Klein, bebe stores, Aeropostale, Monsoon Accessorize, Dooney & Bourke, JC (Jeans and Clothes), European clothing line Morgan de Toi, Jaspal, and Neutrogena skincare products. She became the spokesperson for Keds Sneakers, substantially increasing their sales. In Australia, she appears in commercials for teen magazine Famous and was the guest of honour at retail giant David Jones Tahitian Summer Collection launch. She has also endorsed Chanel jewelry, appearing at a number of events for the brand. In a 2005 interview, she stated that she liked the Chanel brand because they produced a large number of clip-on earrings suitable for women who did not have pierced ears, as she did not at the time. In 2009, having had her ears pierced especially for her role in The Beautiful Life, she modeled jewelry for the company Inspired Creations, including pierced earrings and their Waxing Poetic range of charms.
In 2008, she won the InStyle "icon of the year" award from Karl Lagerfeld at the Viper awards in Berlin. Lagerfeld spoke about Barton; "So many girls want to look like Mischa Barton. She is a fashion role model for an entire generation. I am so happy to be able to present her with the VIPer Award."
Barton also became a spokesperson for the SAFE (Skin Awareness for Everyone) campaign in 2007. In addition, she lent her name to the TRAID campaign (Textile Recycling for Aid and International Development) in association with Visa Swap. Barton was photographed in April 2007 for the Fall 2007/2008 advertising campaign for Italian clothing brand Iceberg, modelling with Nicolas Bemberg.
In July 2008, she created a 14-piece handbag collection for online retailer, ASOS.com. Barton was reportedly very involved in the line which retails from £20 to £145. She told ASOS magazine that she first discovered the website through her sister.
In December 2008, Barton premiered her line of headbands released on the Stacy Lapidus line. The headbands cost from $90 to $200. She also has a handbag line for London-based Ri2k, which is sold in the United Kingdom and Australia only. In early 2009, she became the "new face" of Herbal Essences.
In June 2010 she was announced as the face of Philipp Plein couture. Plein revealed, “Being in the limelight for a young and beautiful girl, the looks of course always get a lot of attention, Mischa Barton got a lot and learned to live with her fame. For me it was the big challenge to again show her as a Diva, a Rock ´n` Roll Goddess”. The campaign will be circulated in international editions of leading fashion magazines.
In October 2011 she appeared in a controversial photoshoot for Tyler Shields that was showcased at the Imitate Modern Gallery in London. Artinfo remarked at the "sense of subjugation" in the raw-meat inspired shoot, "Barton is naked, lipsticked, and icily coiffed like a Duran Duran-era object of desire."
In December 2011 it was reported that Barton plans to open a clothes and shoes store in Shoreditch, the store has the working-title of 'Mischa's Closet'. In March 2012 she debuted a new line of clothing, accessories, cosmetics, and bath products on the internet called Mischa's Place. Barton has plans to expand Mischa's Place beyond the internet and is set to open a Mischa's Place flagship store in London at Spitalfields Market.
In November 2012, she became the face of UUNIQUE LONDON, which makes bespoke iPhone cases and also launched her own range.
Barton purchased a house in Los Angeles at age nineteen, which she shares with her parents; she splits her time between there and apartments in London and New York.
Barton's previous boyfriends have included Cisco Adler, Taylor Locke, Jamie Dornan, Brett Simon, and Brandon Davis (grandson of Marvin Davis). In mid-2007, she briefly dated magician Brandon Silverfield, making a number of appearances as a special guest assistant in his Las Vegas magic show being beheaded by a guillotine and sawed in half in his "Impossible Sawing" illusion. In December 2008, there were reports that she was dating The Kooks frontman Luke Pritchard. The reports were confirmed during the relationship by Pritchard; Barton only confirmed the relationship after the pair split in March 2009, writing on her blog: "I think Luke is a great guy, but the type better suited as a friend. We have a surprising amount of mutual friends in England and I don't regret our time together, I just couldn't keep it going."
Barton is currently the spokesperson for Climate Star, an organization that fights global warming through social and legislative activism. Barton has worked with the home shopping company QVC to raise funds for women's cancer research.
In 2006, Barton also became a spokesperson for the SAFE (Skin Awareness for Everyone) campaign and also lent her name to the TRAID campaign (Textile Recycling for Aid and International Development) in association with Visa Swap. Barton is an ambassador for Save the Children and the "One Water" campaign which brings water to remote locations in Africa, and is on the board of the "Lupus Research Committee" in Los Angeles.
Barton was arrested on suspicion of driving under the influence, possession of marijuana, and driving without a valid license on 27 December 2007. She was stopped by the police during the early morning hours in West Hollywood, California when she was seen "straddling the lanes and failing to signal for a turn". Police discovered during the traffic stop that Barton "was an unlicensed driver and was driving while under the influence of an alcoholic beverage". She was detained and released later the same morning from the West Hollywood Sheriff Station on US$10,000 bail. On 11 January 2008, Barton called into Ryan Seacrest's syndicated radio show On Air with Ryan Seacrest and said, "I was pulled over... just that. Obviously, I'm 100 percent responsible for my actions in this case and I'm really disappointed in myself... I don't know what to say about it, except that I'm not perfect and I just don't ever intend to do something this stupid again."
Barton was subsequently charged with two misdemeanors: driving without a valid license; and a DUI.
On 21 January 2010, Barton was sued by her New York landlord for refusing to pay three months of $7,000 per month rent on her apartment.
In August 2015, Barton sued her own mother for "breach of contract" and "theft" over a house they own in Beverly Hills, California.
In July 2009, after seeking medical attention for a tooth infection, Barton was sectioned for two weeks under a 5150 that allows a psychiatrist to involuntarily confine a person deemed to have a mental disorder that makes them a danger to themselves and others. She said of the incident, "If they feel you are depressed or a danger to yourself they can hold you on a 5150. I am terrified of needles and they wanted to pump me full of drugs and I said, "No, absolutely not. I don't want to be here," and got into a fight with the nurses, and that led to my 5150."