He originally wrote Sword Art Online in 2001, with the intent to submit it for the 2002 Dengeki Game Novel Prize. However, he was unable to cut it to meet the page limit, and opted instead to publish it online. Inspired by the positive feedback, he went on to write multiple side stories and sequels. Years later, he submitted Accel World for the same contest, and in 2008 he won the Grand Prize. The first volume was published by ASCII Media Works on February 10, 2009. In a meeting in 2008, Kawahara's editor asked to read the drafts of Sword Art Online. After reading them in a week, he decided to publish SAO as well, and it began publication April 2009. He also writes Sword Art Online: Progressive, a reboot of SAO's Aincrad arc, as well as The Isolator, a science-fiction series.
Since his professional debut, Kawahara has enjoyed an immensely popular career. Accel World has spawned two manga adaptations, a two-cour anime adaptation, an anime film, and two games. Sword Art Online has spawned two spinoff light novels, eighteen unique manga titles, four two-cour anime seasons, an anime film, seven console games, five mobile games, and even a board game. The Isolator has also spawned a manga adaptation.