Larsson's first fiction writing efforts were not in crime fiction, but in science fiction. An avid science fiction reader from an early age, he became active in Swedish science fiction fandom around 1971, co-edited with Rune Forsgren his first fanzine, Sfären, in 1972, and attended his first SF convention, SF?72 in Stockholm. Throughout the 1970s, Larsson published around 30 additional fanzine issues, and after his move to Stockholm in 1977, he became active in the Scandinavian SF Society, where he was a board member in 1978 and 1979, and chairman in 1980. In his first fanzines, 1972—1974, he published a handful of early short stories while submitting others to other semi-professional or amateur magazines. He was co-editor or editor of several fanzines, including Sfären and FIJAGH!; in 1978-1979 he was president of the largest Swedish science fiction fan club, Skandinavisk Förening för Science Fiction (SFSF).

In early June, 2010, manuscripts for two such stories, as well as fanzines with one or two others, were noted in the Swedish National Library (to which this material had been donated a few years earlier, mainly by the Alvar Appeltofft Memorial Foundation, which works to further science fiction fandom in Sweden). This discovery of what was called "unknown" works by Larsson caused considerable publicity.

Larsson's first name originally was Stig which is the standard spelling. In his early twenties, he changed it to avoid confusion with his friend Stig Larsson, who would go on to become a well-known author well before Stieg did. At the time, they were amateur photographers and it was in this capacity that they wished to prevent any misunderstanding; neither had yet published a book. Stieg Larsson, in later years, would tell the story that the two men had tossed a coin to decide who was to change his name, but this is disputed by Stig Larsson. The pronunciation is the same regardless of spelling.

Larsson was initially a political activist for the Kommunistiska Arbetareförbundet (Communist Workers League), a photographer, and one of Sweden's leading science fiction fans. In politics he was the editor of the Swedish Trotskyist journal Fjärde internationalen. He also wrote regularly for the weekly Internationalen. He worked as a graphic designer at the largest Swedish news agency, Tidningarnas Telegrambyrå (TT) between 1977 and 1999.

Larsson's political convictions, as well as his journalistic experiences, led him to the founding of the Swedish Expo Foundation, similar to the British Searchlight Foundation, established to "counteract the growth of the extreme right and the white power-culture in schools and among young people." He also became the editor of the foundation's magazine, Expo. Larsson quickly became instrumental in documenting and exposing Swedish extreme right and racist organizations; he was an influential debater and lecturer on the subject, reportedly living for years under death threats from his political enemies.

At his death, Larsson left behind manuscripts of three completed but unpublished novels in a series. He wrote them for his own pleasure after returning home from his job in the evening, making no attempt to get them published until shortly before his death. The first was published in Sweden in 2005 as Män som hatar kvinnor ("Men who hate women"), published in English as The Girl with the Dragon Tattoo. It was awarded the prestigious Glass Key award as the best Nordic crime novel in 2005. His second novel, Flickan som lekte med elden (The Girl Who Played with Fire), received the Best Swedish Crime Novel Award in 2006. The third novel in the Millennium trilogy, Luftslottet som sprängdes ("The air castle that was blown up"), published in English as The Girl Who Kicked the Hornets' Nest, was published in the United States in May 2010.

Larsson left about three quarters of a fourth novel on a notebook computer, now possessed by his partner, Eva Gabrielsson; synopses or manuscripts of the fifth and sixth in the series, which he intended to contain an eventual total of ten books, may also exist.

The Swedish film production company Yellow Bird has produced film versions of the Millennium Trilogy, co-produced with The Danish film production company Nordisk Film and TV company, which were released in Scandinavia in 2009.

Larsson died in Stockholm at the age of 50 of a heart attack. Rumours that his death was in some way induced, because of death threats received as editor of Expo, have been denied.

In May 2008 it was announced that a 1977 will, found soon after Larsson's death, declared his wish to leave his assets to the Umeå branch of the Communist Workers League (now the Socialist Party). As the will was unwitnessed, it was not valid under Swedish law, with the result that all of Larsson's estate, including future royalties from book sales, went to his father and brother. His long term lover Eva Gabrielsson, who found the will, has no legal right to the inheritance, sparking controversy between her and his father and brother. Reportedly, the two never married because, under Swedish law, couples entering into marriage are required to make their addresses (at the time) publicly available; marrying would have been a security risk. Owing to his reporting on extremist groups and the death threats he had received, the couple had sought and been granted masking of their addresses, personal data and identity numbers from public records, to make it harder for others to trace them; this kind of "identity cover" was integral to his work as a journalist and would have been difficult to bypass if the two had married or become registered partners.

An article in Vanity Fair magazine discusses Gabrielsson's dispute with Larsson's relatives, which has also been well covered in the Swedish press. She claims the author had little contact with his father and brother and requests the rights to control his work so it may be presented in the way he would have wanted. Larsson's story was featured on the October 10, 2010 segment of CBS News Sunday Morning. In this segment Larsson's family claims the fourth and as yet unpublished book is actually the fifth book.

Through his written works, as well as in interviews, Larsson acknowledged that a significant number of his literary influences were American and British crime/detective fiction authors. His heroine has some similarities with Carol O'Connell's "Mallory," who first appeared in "Mallory's Oracle" in 1994. In his work he made a habit of inserting the names of some of his favourites within the text, sometimes by making his characters read the works of his own favorite authors. Topping the list were Sara Paretsky, Agatha Christie, Val McDermid, Dorothy Sayers and Enid Blyton. One of the strongest influences originates from his own country: Pippi Longstocking, by Sweden's much-loved children's author Astrid Lindgren. Larsson explained that one of his main recurring characters in the Millennium series, Lisbeth Salander, is actually fashioned on a grown-up Pippi Longstocking as he chose to sketch her.

When Larsson was 14 years old he witnessed his friends gang rape a girl, which led to his lifelong abhorrence toward violence and abuse against women. The author never forgave himself for failing to help the girl, whose name was Lisbeth - like the young heroine of his books, who is also a rape victim. This inspired the themes of sexual violence against women in his books.

There are also similarities between Larsson's Lisbeth Salander and Peter O'Donnell's Modesty Blaise.[1][2][3][4] Both are women from disastrous childhoods who somehow survive to become adults with notable skills, including fighting, and who accomplish good by operating somewhat outside the law. One of Larsson's villains, Ronald Niedermann (a.k.a. "blond hulk"), has much in common with the invulnerable, sociopathic giant named Simon Delicata in the fourth Modesty Blaise book A Taste for Death.

Non-fiction books

• Stieg Larsson, Anna-Lena Lodenius: Extremhögern, Stockholm, 1991
• Stieg Larsson, Mikael Ekman: Sverigedemokraterna: den nationella rörelsen, Stockholm, 2001
• Stieg Larsson, Cecilia Englund: Debatten om hedersmord: feminism eller rasism, Stockholm, 2004
• Richard Slätt, Maria Blomquist, Stieg Larsson, David Lagerlöf m.fl.: Sverigedemokraterna från insidan, 2004

Novels

The Millennium series:

• Män som hatar kvinnor ("Men Who Hate Women"), 2005. English translation by Reg Keeland under the title The Girl with the Dragon Tattoo, January 2008. US release September 16, 2008.
• Flickan som lekte med elden ("The Girl Who Played with Fire"), 2006. English translation by Reg Keeland under the title The Girl Who Played with Fire, January 2009. US release July 28, 2009.
• Luftslottet som sprängdes ("The Air Castle that was Blown up"), 2007. English translation by Reg Keeland under the title The Girl Who Kicked the Hornets' Nest, October 2009 (UK release). US release date May, 2010.

Periodicals edited

• Sfären, FIJAGH! and other science fiction fanzines, 1972 and later
• Svartvitt med Expo, 1999—2002
• Expo, 2002—2004

• 2006 - Glass Key award, Män som hatar kvinnor
• 2006 - Best Swedish Crime Novel Award, Flickan som lekte med elden
• 2008 - Glass Key award, Luftslottet som sprängdes
• 2008 - ITV3 Crime Thriller Award for International Author of the Year, UK, for The Girl with the Dragon Tattoo.
• 2008 - Exclusive Books Boeke Prize, South Africa, for The Girl with the Dragon Tattoo
• 2009 - Galaxy British Book Awards, Books Direct Crime Thriller of the Year, UK, for The Girl with the Dragon Tattoo
• 2009 - Anthony Award, Best First Novel, for The Girl with the Dragon Tattoo
• 2009 - General Council of the Judiciary, Spain, for his contribution to the fight against domestic violence.

Stieg Larsson is the first author to sell more than one million e-books on Amazon.com.

Books

• Barry Forshaw, The Man Who Left Too Soon: the Biography of Stieg Larsson, John Blake, 2010.

Journalism

• Expo's memorial page for Stieg Larsson /
• Stieg Larsson: The Man Who Died Too Soon, Val McDermid, Waterstones Book Quarterly, October 2009
• In praise of Stieg Larsson, The Guardian, Culture Editorial, 3 October 2009
• In search of Stieg Larsson BBC News Magazine Feature, 28 January 2010
• Thriller with a radical message, Socialist Worker, 15 April 2010