Koiso graduated from the Army Academy in 1900 at the top of his class, attended the Army War College, and served on active duty during the Russo-Japanese War. In 1930 he became chief of the Bureau of Military Affairs and in 1932 was appointed vice-minister of war. He later became commander of the 5th division and chief of staff of the Kantōgun, the military operation in China. From 1935 to 1938, Koiso, as commander in chief, directed military operations in Korea. During the Hiranuma (1939) and Yonai (1940) cabinets, he served as minister of overseas affairs.

At the onset of World War II Koiso returned to Korea as governor-general. After the fall of the Tōjō cabinet in 1944, he was appointed prime minister to carry on the war effort. He resigned in April 1945 as U.S. troops landed in Okinawa. He was convicted of war crimes and died while serving a life sentence.