In 1934 he was part of the staff of Isoroku Yamamoto during the preparation of the Japanese delegation for the London naval conference in which he participated as an attendant.

He was probably chosen as a participant because he was aware of the European mentality with which he had become familiar as early as the 1930s when he was a naval attache at the Japanese Embassy in Paris.

During the later stages of World War II Mitunobu was posted to Italy as a naval attaché to the Japanese embassy.

In Pianosinatico (close to Abetone pass) along the Gothic Line, his car was stopped by Italian partisans, commanded by Manrico "Pippo" Ducceschi, and was killed as he tried to escape.

Important documents concerning the War in the Pacific were found, which helped to enable the United States and the Allies in their later manoeuvers.