讃岐国
Tadamitsu was the second son of Kyōgoku Takatoyo, daimyō of Marugame Domain in Sanuki Province. As Nishio Tadanao, had no heirs, he adopted his nephew in 1729. In 1731, Tadamitsu received the courtesy title of "Mondo no Shō" (主水正) along with junior 5th court rank, lower grade ("ju go i no ge" 従五位下). Tadamitsu became head of the Nishio clan in 1760, on Tadanao's death, and was appointed "Sōshaban" (master of ceremonies) in the administration of the Tokugawa shogunate. He retired in 1782, and was succeeded by his son Tadayuki.