Yokosuka Domain
Tadanao was the fourth son of Nishio Tadanari, the first daimyō of the Nishio clan at Yokosuka Domain. He was recognized as heir apparent in 1696 and granted courtesy rank and title of junior 5th court rank, lower grade ("ju go i no ge" 従五位下), and "Harima no Kami" in 1703. He became clan leader upon his father's retirement in 1713. In the same year, he received his father's former courtesy title, "Oki no Kami".