From 1911-1913, he left active service to assume his seat in the House of Peers, returning in December 1914 to the battleship Settsu after having completed navigation and advanced gunnery training.

On his return, he was appointed to the Imperial Japanese Navy General Staff and became an instructor at the Naval War College.

After his return to Japan, Shimazu served on the Navy General Staff, and was promoted to rear admiral on 15 November 1935.

In the postwar era, he was involved in a number of financial scandals, notably with regards to the Shimazu Kosan Company.

He was forced to sell off most of his inheritance to make ends meet, including the Shimazu clan archives, which went to Tokyo University.