He edited the Labour Party newspapers "Demokraten" from 1920 to 1927, and "Hamar Arbeiderblad" from 1927 to 1935. In 1935 he was hired as a journalist in "Arbeiderbladet". In 1940, when Norway became invaded and occupied by Germany, Larssen was the acting news editor of "Arbeiderbladet" before it was stopped by the Germans. He co-edited the illegal "Bulletinen" from 1940 to 1942, but for this he was imprisoned. He was held at Møllergata 19 from January to April 1942, then at Grini until February 1943, and thereafter in the Sachsenhausen concentration camp until the war ended. From this period he wrote the section "Blant landsmenn i Sachsenhausen" ('Among fellow countrymen in Sachsenhausen') in the memoir book "3 fra Sachsenhausen" (together with August Lange, Carl Johan Frederik Jakhelln and W. Winiarski).