Yonekoto Masakoto was the 6th son of the 7th "daimyō" of Mutsuura Domain, Yonekura Masanaga. In May, 1868, he was presented before "Shōgun" Tokugawa Iesada in a formal audience and on June 24, 1860, due to his father's retirement due to illness, became the head of the Yonekura clan, and "daimyō" of Mutsuura Domain. He served in a number of ceremonial posts within the administration of the Tokugawa shogunate, including guard duty at Osaka Castle, and participated in both the First Chōshū expedition and the Second Chōshū expedition. In 1867, he was assigned guard duty over the Yokosuka Naval Arsenal; however in 1868 during the Boshin War of the Meiji Restoration, he capitulated to the Satchō Alliance forces without a struggle. For this reason, he was confirmed as domain governor of Mutsuura in June 1869. He left public service with the abolition of the han system in July 1871.