When the Germans invaded Norway in April 1940, Motzfeldt was in the United States in order to acquire more aircraft for the Norwegian Army Air Service. He was subsequently appointed assistant air attaché in the US. Among his initiatives was the establishing of the pilot training camp Little Norway. He returned to London in 1941, and took part in the development of the Royal Norwegian Air Force in exile in Great Britain. He was also involved in the air route between Stockholm and Scotland during the war. During the Cold War he contributed to the development of a modernized air force in Norway. He was promoted to the rank of major general in 1953, and to lieutenant general in 1955. He was appointed head of the Royal Norwegian Air Force from 1955 to 1960, succeeding fellow World War II veteran Finn Lambrechts. The Royal Norwegian Air Force went through significant development during his leadership.