During World War II Grahamslaw joined Australian New Guinea Administrative Unit and was appointed as a district officer for Northern District. Having walked from the north to the south coast of New Guinea alone following the Japanese invasion, he helped prepare Australian soldiers for the Kokoda Track campaign. He was temporarily promoted to major and was awarded an OBE in 1943 for his efforts during the Battle of Buna–Gona. In 1943 he transferred to the Australian Imperial Force, and in early 1944 was given responsibility for Lakekaum district, before taking over the administration of a much larger area later in the year. In 1945 he was promoted to lieutenant colonel, remaining in the army until being demobilised in February 1946.