Mutō was a native of Hakusui, Kumamoto, and a graduate of the 25th class of the Imperial Japanese Army Academy in 1913.

Mutō was recalled to Japan in 1939, promoted to major general in 1939, and served on the Military Affairs Bureau of the Ministry of War.

[citation needed] Promoted to lieutenant general just prior to the start of the Pacific War, Mutō served as director of the Military Affairs Bureau at the time of the attack on Pearl Harbor.

After the surrender of Japan, Mutō was arrested by the American occupation authorities and charged with war crimes before the International Military Tribunal for the Far East.

He was convicted for atrocities against civilians and prisoners of war in both China and the Philippines, and was executed by hanging on 23 December 1948.