A daimyo of Dewa who became the head of the Mogami Clan at the age of three. The Mogami Clan at this time was a puppet of the Date Clan, but due to internal strife in the Date Clan around 1542, the Mogami Clan was able to break free.

In his latter years, Yoshimori doted on his second son and alienated his heir, Mogami Yoshiaki. Mogami Yoshiaki planned to stage a coup d'etat, but Date Terumune interfered. The vassals of the Mogami intervened to force Yoshimori to hand his position over to Yoshiaki and retire.