In 1917, Marumo was assigned to the second Special Task Fleet under Admiral Kozo Sato, a task force of Japanese destroyers deployed to Malta in the Mediterranean as part of Japan’s assistance to the Allied war effort under the Anglo-Japanese Alliance.

He subsequently served as chief gunnery officer on the cruiser Nisshin and battleships Ise and Hyūga.

After his promotion to lieutenant commander in 1924, he was assigned to the cruiser Yakumo, battleship Yamashiro and destroyer Kako.

In 1929, he made commander, and joined the Imperial Japanese Navy General Staff in a number of shore assignments.

[2] From December 1942, Marumo was assigned as naval attaché to the Empire of Manchukuo, where he spent most of the remainder of the war.