Tokugawa Ienari
Hachisuka was born at the Hachisuka domain residence in Edo, as the eldest son of the 13th "daimyō" Hachisuka Narihiro (1821–1868). Hachisuka Narihiro was the 22nd child of "shōgun" Tokugawa Ienari, and was adopted into the Hachisuka clan as the 12th "daimyō", Hachisuka Narimasa was childless. Thus, Mochiaki was the grandson of Tokugawa Ienari, nephew of Tokugawa Ieyoshi and cousin of Tokugawa Iesada and Iemochi.