Kase was born in Chiba, Japan. After passing his Foreign Service Examination in 1925 he left Tokyo Higher Commercial College (later Hitotsubashi University) and attended Amherst College and Harvard as a Research Fellow, graduating in 1927. He took up diplomatic posts in both Berlin and London before returning to Tokyo where he was posted to the North America desk of the Japanese Foreign Office. He was on duty on the weekend of the Pearl Harbor attack in December 1941. Kase was present as part of the Japanese delegation on board USS Missouri for the signing of the treaty of surrender in 1945. In 1950 Kase published a book which gave an account of the war from a Japanese perspective. In 1955 he became Japan's first ambassador to the United Nations. He died, aged 101 years, in Kamakura of heart failure.