Lagercrantz studied philosophy and religion at university and subsequently graduated from the Gothenburg journalism school. His first journalist job was at the in-house magazine of carmaker Volvo. He later moved to the daily tabloid newspaper Expressen and worked until 1993 as a crime reporter, covering some of the major criminal cases of the late 80s and early 90s in Sweden, notably the Åmsele murders. His first book was released in 1997, a biography of the Swedish adventurer and mountaineer Göran Kropp (1966 - 2002).