Stewart was born and raised in Los Angeles, California. Her father, John Stewart, is a stage manager and television producer who has worked for Fox. Her mother, Jules Mann-Stewart, is a script supervisor originally from Maroochydore, Queensland, Australia. She has an older brother, Cameron Stewart, and an adoptive brother, Taylor. Stewart attended school until the seventh grade, and then continued her education by correspondence. She has since completed high school.
Her whole family worked behind the camera, and Stewart thought she would become a writer/director, but never considered being an actress. "I never wanted to be the center of attention - It wasn't that I want to be famous, I want to be an actor kid. I never sought out acting, but I always practiced my autograph because I love pens. I'd write my name on everything." Stewart's acting career began at the age of eight, after an agent saw her perform in her elementary school's Christmas play. Stewart's first role was a nonspeaking part in the film The Thirteenth Year. Then, she had another part in the film The Flintstones in Viva Rock Vegas as the "ring toss girl". She subsequently appeared in the independent film The Safety of Objects, in which she played the tomboy daughter of a troubled single mother. Stewart had a major role in the Hollywood film Panic Room, playing the sullen, diabetic daughter of a divorced mother. The film received generally positive reviews, and Stewart garnered positive notices for her performance.
After Panic Rooms success, Stewart was cast in another thriller, Cold Creek Manor, playing the daughter of Dennis Quaid's and Sharon Stone's characters; the film generally failed at the box office. Her first starring role followed, in the children's action-comedy Catch That Kid, opposite Max Thieriot and Corbin Bleu. Stewart also played the role of Lila in the thriller Undertow. To date, Stewart's most critically acclaimed role may be in the television film Speak (2004), based on the novel by Laurie Halse Anderson. Stewart, 13 at the time of filming, played high school freshman Melinda Sordino, who stops almost all verbal contact after being raped and who deals with enormous amounts of emotional turmoil. Stewart received great praise for playing the character, who had only a few speaking lines, but kept up a dark-witted commentary inside her head throughout the film.
2005, Stewart appeared in the fantasy-adventure film Zathura, playing the role of Lisa, the irresponsible older sister of two little boys, who turn their house into a spacecraft hurtling uncontrollably in outer space by playing a board game. The film received praise by critics, but Stewart's performance did not garner much media attention, as it was noted that her character is immobilized during most of the film. The following year, she played the character Maya in Fierce People, directed by Griffin Dunne. After that film, she received the lead role of Jess Solomon in the supernatural thriller film The Messengers.
In 2007, Stewart appeared as teenager Lucy Hardwicke in In the Land of Women, a romantic drama starring Meg Ryan and The O.C. star Adam Brody. The film, as well as Stewart's performance, received mixed reviews. That same year, Stewart starred in Sean Penn's critically acclaimed adaptation film Into the Wild. For her portrayal of Tracy — a teenage singer who has a crush on young adventurer Christopher McCandless — Stewart received generally positive reviews. Salon.com considered her work a "sturdy, sensitive performance", and the Chicago Tribune noted that she did "vividly well with a sketch of a role." Her performance was not without detractors, however; Varietys critic Dennis Harvey wrote, "It's unclear whether Stewart means to be playing hippie-chick Tracy as vapid, or whether it just comes off that way." After Into the Wild, Stewart had a cameo appearance in Jumper and also appeared in What Just Happened, which was released in October 2008. She also co-stars in The Cake Eaters and The Yellow Handkerchief, both independent films that have only been screened at film festivals.
On November 16, 2007, Summit Entertainment announced that Stewart would play Isabella "Bella" Swan in the film Twilight, based on Stephenie Meyer's bestselling vampire romance novel of the same name. Stewart was on the set of Adventureland when director Catherine Hardwicke visited her for an informal screen test which "captivated" the director. She stars alongside Robert Pattinson, who plays Edward Cullen, her character's vampire boyfriend. The film began production in February 2008 and finished filming in May 2008. Twilight was released domestically on November 21, 2008. After the release of Twilight, Stewart was awarded the MTV Movie Award for Best Female Performance for her portrayal as Bella Swan. Stewart reappeared as Bella in the sequels The Twilight Saga: New Moon, The Twilight Saga: Eclipse and The Twilight Saga: Breaking Dawn parts 1 and 2.
Stewart was also cast to portray Joan Jett in The Runaways, a biopic of the titular band from writer-director Floria Sigismondi. Stewart met with Jett over the 2008-2009 New Year to prepare for the role, and hopes to be able to sing in the film.
In September 2011, Stewart made in an appearance in Marcus Foster's music video for "I Was Broken".
Stewart currently lives in Woodland Hills in Los Angeles, California. She has expressed a desire to live and work in Australia, saying, "I want to go to Sydney University in Australia. My mom's from there." Apart from acting, she is also interested in attending college in the near future, saying, "I want to go to college for literature. I want to be a writer. I mean, I love what I do, but it's not all I want to do — be a professional liar for the rest of my life." Stewart also enjoys playing the guitar and singing when she can. In an interview with Vanity Fair, Stewart stated that she dated Michael Angarano, her co-star from Speak. Stewart was the centre of controversy when she was photographed Rupert Sanders, the married director of Snow White and the Huntsman. This lead to break-down of Sanders' marriage and Stewart's relationship with Twilight co-star Robert Pattinson. She is now dating her former assistant, Alicia Cargile.