Shortly after the "real" Toshimochi died, he was renamed and brought to Edo to take his place, and was received in formal audience by shōgun Tokugawa Ienari on 15 November 1821.

He was confirmed as daimyō of Morioka, and received the courtesy title of Daizen-no-taifu and junior 4th court rank, lower grade in December of the same year.

Due to his age and the circumstances of his accession, he was unable to quell the factionalism which plagued the domain's politics.

Furthermore, the Sōma Daisaku Incident, in which a former Nanbu samurai attempted to assassinate the daimyō of Hirosaki Domain, Tsugaru Yasuchika occurred in April 1821.

Fearing that the shogunate might demand his life over the incident, Toshimochi appointed Nanbu Toshitada as his heir.