Munthe-Kaas was the son of Otto H. Munthe-Kaas and younger brother of Otto Ulrik Munthe-Kaas, both military officers. His father was, before World War II, head of the military camp at Setermoen, and was head of the II Field Battalion in the 16th Infantry Regiment during the German invasion of Norway. Hugo Munthe-Kaas had before the outbreak of the war, volunteered for the guard company in Tromsø, where he served during the German attack. He then joined the war commissary in Tromsø and was accepted for service in the 16th Infantry Regiment, despite being two years too young for conscription. Munthe-Kaas served during the campaign in Northern Norway as an orderly in the courier service and as a sniper in the Battles of Narvik. His service lasted until the Norwegian mainland capitulation on 10 June 1940.