On 9 April 1940, Nazi Germany invaded and occupied Norway as a part of World War II. Young Zeiner-Henriksen became a member of the Norwegian Independent Company 1 (). From 1943 to 1944, he co-operated with Max Manus and Oslogjengen and, from 1944 to 1945, he participated in the Special Operations Executive operation GREBE in Dovrefjell and Rondane.