Morozumi was born in Nagano prefecture. He graduated from the 22nd class of the Imperial Japanese Army Academy in May 1905, serving initially with the IJA 58th Infantry Regiment, a new regiment which had been raised as a reserve force in the closing stages of the Russo-Japanese War. As a second lieutenant, he returned to the Army’s Infantry Warfare School, where he specialized in armor. His rise through the ranks was uneventful, serving a staff post with the Inspectorate General of Military Training, and a field posting as battalion commanders within the IJA 59th Infantry Regiment, IJA 1st Infantry Regiment, IJA 29th Infantry Regiment and IJA 65th Infantry Regiments and as commander of the Hongo Regimental District. The IJA 65th Regiment was one of the units at the Battle of Nanking in 1937 during the Second Sino-Japanese War and was accused of the massacre of prisoners of war following the battle's end.