When World War II reached Norway on 9 April 1940, with the German invasion, Utne volunteered and fought for Norway in the battles of Southern Norway. After Norway capitulated, he was a co-founder of the resistance group "Theta".[2] The group was self-initiated in a circle of friends, but they lacked contacts, knowledge and materials to actually conduct intelligence work.[3] Other members of the group running "Theta" were Bjarne Thorsen, Kristian Ottosen,[3] Helmer Dahl,[4] Hagbarth Schjøtt, Jr. and Jan Dahm.[5] Two members were sent to the United Kingdom: Leif Utne and Bjarne Thorsen. Utne travelled via Sweden, while Thorsen crossed the Norwegian Sea.[3] Thorsen returned with the necessary equipment, and a radio post was finally set up in December 1941, with its headquarters at the famous seaside location Bryggen.[6]
By that time, however, Leif Utne had become stationed in Scotland as a part of the Royal Norwegian Navy-in-exile.[2] He operated out of Lerwick in a motor torpedo boat squadron, among others together with Hugo Munthe-Kaas.[7] Utne was decorated with the Norwegian War Medal.[1]