讃岐国
Ōkubo Tadanori was the nephew of Tokugawa Nariaki, which made him a cousin to Shōgun Tokugawa Yoshinobu. He was born as the 5th son of Matsudaira Yorihiro, daimyō of Takamatsu Domain, in Sanuki Province. On the death of former daimyō of Odawara, Ōkubo Tadanao in 1859 he was adopted into the Ōkubo clan as 11th clan head, and by default, daimyō of Odawara Domain. He served in a number of posts within the administration of the Tokugawa shogunate, including that of "Sōshaban" (Master of Ceremonies) in November 1863, and accompanied "Shōgun" Tokugawa Iemochi on his visit to Kyoto in 1864. He was briefly appointed "Kōfu jōdai" from September to December 1867.