During World War II, Tidemand-Johannessen participated in the defense of Norway in 1940, and then in the Norwegian resistance to the German occupation. He was arrested in February 1941 by the Nazi authorities. For a year and a half, between February 1941 and July 1942, he was being moved between the prisons in Åkebergveien and Møllergaten 19. Then, from 9 July 1943 onwards he was imprisoned at Grini concentration camp.