After graduating from the Imperial Japanese Army Academy, he served in the First Sino-Japanese War as a lieutenant in the infantry.

[1] From 1915-1916, Mutō was Chief of 2nd Section (Maneuvers), 1st Bureau, Imperial Japanese Army General Staff.

From 1917, he was assigned to military intelligence and headed the Harbin Special Agency and operational offices in Irkutsk and Omsk.

He returned to administrative positions in Japan from 1919–1921, before being appointed commander of the IJA 3rd Division in 1921 and being dispatched to Russia during the Siberian Expedition against the Bolshevik Red Army.

Mutō returned to Japan in 1922 as Vice Chief of the General Staff to 1925, and a member of the Supreme War Council from 1925-1926.