In the 1930s he was a member of Nasjonal Samling. He left the party in the 1937 party split, continuing though in the "Ragnarok" group around national socialist Hans S. Jacobsen, and edited the magazine "Ragnarok" in 1940. In 1940, however, Norway was invaded and occupied by Nazi Germany. Engelschiøn joined the resistance movement and was the head of intelligence in Milorg's District 13, Division 3 (Asker and Bærum) from 1943 to 1945. He received the Defence Medal 1940–1945 with rosette.