Synnergren was commissioned as an officer with the rank of second lieutenant in 1939 and served in the Ski Battalion (I 19 K) in Kiruna during World War II and guarded the Norwegian border when Germany occupied Narvik. After the German attack on Norway on 9 April 1940, he as an intelligence officer in the regimental staff, on several occasions, alone and on skis, got into the war zone in Northern Norway to make contact with the Norwegian forces and gather intelligence within the German dominated area. Because of his fell experience and skills, he became, after a brief interlude as a cadet officer at Karlberg in 1944, the head of evacuation in Jokkmokk and was as such responsible for management of the Norwegian refugees. After the war, he was appointed captain in 1946 and did in a short time a rapid military career.