Mito Domain
Tadakazu was the 16th son of the Mito Domain Tokugawa Nariaki, and was thus the younger brother of Shōgun Tokugawa Yoshinobu. In 1861, when the 7th "daimyō" of Shimabara, Matsudaira Tadachika died without heir, Tadakazu was selected as his replacement, and was posthumously adopted as his son. He assumed the office of "daimyō" on December 16, 1861 and was awarded lower 6th court rank the same day.