Yokosuka Domain
Tadaatsu was the son of Nishio Tadasaka, the 7th "daimyō" of Yokosuka Domain. His mother was a daughter of Toki Yorinobu, the "daimyō" of Numata Domain in Kōzuke Province. He became "daimyō" of Yokosuka and head of the Nishio clan upon his father's death in 1861 and was received in formal audience by Shogun Tokugawa Iemochi the following year.