During the First Sino-Japanese War, Shimamura was assigned as a staff officer of the Standing Fleet from August 1894 to April 1895 and involved in planning the column formations of the battle.

[3] After serving in various staff positions after the war, (including naval attaché to Italy in 1894) Shimamura was promoted to captain in 1899 and commanded the cruiser Suma and marines during the Battle of Tientsin during the Boxer Rebellion.

Promoted to rear admiral on June 6, 1904, shortly before the Russo-Japanese War, Shimamura was made Chief of Staff of the 1st Fleet.

Shimamura was initially opposed to the deployment of the Imperial Japanese Navy to the Mediterranean under the Anglo-Japanese Alliance, as he felt that this would weaken Japan's defenses against the "true threat" of the United States.

[4] Promoted to full admiral on August 28, 1915, Shimamura was ennobled as a danshaku (baron) under the kazoku peerage system in 1916.