Carlsen was born in Tønsberg, Norway, on 30 November 1990, to Sigrun Øen, a chemical engineer, and Henrik Albert Carlsen, an IT consultant. The family spent one year in Espoo, Finland, and then in Brussels, Belgium, before returning to Norway in 1998, where they lived in Lommedalen, Bærum. They later moved to Haslum. Carlsen showed an aptitude for intellectual challenges at a young age: at two years, he could solve 50-piece jigsaw puzzles; at four, he enjoyed assembling Lego sets with instructions intended for children aged 10–14.
His father, a keen amateur chess player, taught him to play chess at the age of 5, although he initially showed little interest in the game. He has three sisters, and in 2010 he stated that one of the things that first motivated him to take up chess seriously was the desire to beat his elder sister at the game.
The first chess book Carlsen read was a booklet named Find the Plan by Bent Larsen, and his first book on openings was Eduard Gufeld's The Complete Dragon. Carlsen developed his early chess skills by playing by himself for hours on end—moving the pieces around, searching for combinations, and replaying games and positions shown to him by his father. Simen Agdestein emphasises Carlsen's exceptional memory, stating that he was able to recall the areas, population numbers, flags and capitals of all the countries in the world by the age of five. Later, Carlsen had memorised the areas, population numbers, coat-of-arms and administrative centres of "virtually all" Norwegian municipalities. Carlsen participated in his first tournament—the youngest division of the 1999 Norwegian Chess Championship—at the age of 8 years and 7 months, and scored 6½/11.
Carlsen was coached at the Norwegian College of Elite Sport by the country's top player, Grandmaster (GM) Simen Agdestein, who in turn cites Norwegian Egil "Drillo" Olsen as a key inspiration for his coaching strategy. In 2000, Agdestein introduced Carlsen to Torbjørn Ringdal Hansen, a former Norwegian junior champion and later International Master (IM) and Grandmaster (GM), as Ringdal served a one-year siviltjeneste (an alternative civilian service programme) at the college. Over the course of this year, Carlsen's rating rose from 904 in June 2000, to 1907. Carlsen's breakthrough occurred in the Norwegian junior teams championship in September 2000, where Carlsen scored 3½/5 against the top junior players of the country, and a performance rating (PR) of about 2000. Apart from chess, which he studied about three to four hours a day, Carlsen's favourite pastimes included playing and reading Donald Duck comics. Carlsen also practised skiing until the age of ten.
From autumn 2000 to the end of 2002, Carlsen played almost 300 rated tournament games, as well as several blitz tournaments, and participated in other minor events. In October 2002, he placed sixth in the European Under-12 Championship in Peñiscola. In the following month, he tied for first place in the World Under-12 Championship in Heraklio, placing second to Ian Nepomniachtchi on tiebreak. After this, he obtained three IM norms in relatively quick succession; his first was at the January 2003 Gausdal Troll Masters (score 7/10, 2453 PR), the second was at the June 2003 Salongernas IM-tournament in Stockholm (6/9, 2470 PR), and the third and final IM norm was obtained at the July 2003 Politiken Cup in Copenhagen (8/11, 2503 PR). He was officially awarded the IM title on 20 August 2003. After finishing primary school, Carlsen took a year off to participate in international chess tournaments held in Europe during the fall season of 2003, returning to complete secondary education at a sports school. During the year away from school, he placed joint-third in the European Under-14 Championship and ninth in the World Under-14 Championship.