In August 1897, Katō was assigned as secretary to Navy Minister Saigō Tsugumichi, who was soon promoted to fleet admiral.

Katō returned to sea duty from December 1898 to April 1901, serving as executive officer on the cruisers Akitsushima and Kasagi.

From April 1901, he served with the Imperial Japanese Navy General Staff, where he enjoyed the confidence of Admiral Itō Sukeyuki.

At the start of the Russo-Japanese War, Katō was captain of the cruiser Hashidate, an obsolete vessel not regarded as capable for front-line combat duties.

After the end of the war, Katō served for ten months in the Naval Personnel Department before returning to sea to captain the cruiser Izumo, followed by the battleships Kashima and Iwami.

At Tsingtao, Katō commanded a fleet of four battleships, two cruisers, 15 destroyers along with submarines, torpedo boats and other auxiliary vessels.

Katō was especially vehement in his opposition to the Washington Naval Treaty and was thus a prominent member of the Fleet Faction within the Imperial Japanese Navy.