During the Russo-Japanese War, Katō served as chief of staff to Admiral Tōgō Heihachirō on the battleship Mikasa, assisting in Japan's victory at the Battle of Tsushima.

During this time, he suffered from a very weak stomach, and was vomiting as he issued orders throughout the battle, despite having taken large amounts of medication.

Under Hara and Takahashi, Katō was Japan's chief commissioner plenipotentiary to the Washington Naval Conference, and worked with Ambassador Shidehara Kijurō in the negotiations that led to the Five-Power Treaty.

His cabinet consisted mainly of bureaucrats and members of the House of Peers, which proved unpopular with the Imperial Japanese Army.

Katō was given the honorary rank of Marshal Admiral the day before his death, and posthumously awarded the Grand Cordon of the Supreme Order of the Chrysanthemum and his title raised to shishaku (viscount).