Shimomura was born in Kōchi Prefecture, but was raised in Kanazawa, Ishikawa, where his father was a recruiting officer for the Imperial Japanese Army.

After attended military preparatory schools in Kanazawa and Nagoya, he graduated 6th out of 273 cadets from the 20th class of the Imperial Japanese Army Academy in 1908.

He was assigned as the Army observer to the Japanese delegation at the Geneva Naval Conference negotiations from 1928 to 1929, during which time he was promoted to lieutenant colonel.

He was given a field command in October 1942 in the form of the Japanese Thirteenth Army, a garrison force based in Shanghai and surrounding provinces primarily to deter the possible landings of the Allies of World War II in the lower Yangtze River area of east central China.

In March 1944, Shimomura was withdrawn to the Japanese home islands and became commander of the Western District Army, another force intended to defend against Allied landings.

[1] In May 1945, Shimomura was promoted to full general, and on 23 August (just after the surrender of Japan), was appointed final Army Minister under the Shidehara cabinet.