Kammala jumla mai zuwa: An haifi Larsen a Tilsted, kusa da Thisted a Denmark kuma ya yi karatu a Makarantar Cathedral ta Aalborg. A cikin Janairu 1942 Larsen ya kamu da cututtukan yara da yawa. Kodayake babu wanda ke da tasiri na dindindin, a wannan lokacin ne Larsen ya gano chess. Ya ci gaba da wakiltar Denmark sau biyu a Gasar Zakarun Duniya, a 1951 a Birmingham (na biyar) kuma a 1953 a Copenhagen (na takwas). Ya fara wasa da gaske yana da shekaru 17 lokacin da ya koma Copenhagen don yin karatun injiniyan injiniya, amma bai taɓa kammala karatu ba, maimakon haka ya zaɓi yin wasan chess a sana'a. Ya zama Jagora na Duniya yana da shekaru 19 a 1954, daga aikinsa na tagulla a kan jirgin daya a
Wasannin Olympics na Amsterdam. Ya lashe na farko daga cikin gasar zakarun Denmark guda shida a shekarar 1954, yana maimaita wannan aikin a 1955, 1956, 1959, 1963 da 1964.