On his twelfth birthday, Larsson's parents gave him a typewriter as a birthday gift.
Larsson's first efforts at writing fiction were not in the genre of crime, but rather science fiction. An avid science fiction reader from an early age, he became active in Swedish science fiction fandom around 1971; co-edited, together with Rune Forsgren, his first fanzine, Sfären, in 1972; and attended his first science fiction convention, SF•72, in Stockholm. Through the 1970s, Larsson published around 30 additional fanzine issues; 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, 1972–74, 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 science fiction fanzines, including Sfären and FIJAGH!; in 1978–79, he was president of the largest Swedish science-fiction fan club, Skandinavisk Förening för Science Fiction (SFSF). An account of this period in Larsson's life, along with detailed information on his fanzine writing and short stories, is included in the biographical essays written by Larsson's friend John-Henri Holmberg in The Tattooed Girl, by Holmberg with Dan Burstein and Arne De Keijzer, 2011.
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 generated considerable publicity.