Lifespan: 14xx to 2/26 of Bunmei 12 (1480)
Rank: bushō, daimyō
Title: Master of the Eastern Capital Office
Bakufu: Muromachi – Commissioner of Dewa (?)
Father: Mogami Mitsuie or Mogami Yoshiharu
Adopted Children: Mitsuuji
Mogami Yoshiaki served as a bushō and daimyō during the late Muromachi and early Sengoku periods. As the lord of Yamagata Castle and sixth head of the Mogami clan of Dewa Province, he carried an equivalent rank to a shugo daimyō. He is surmised to have also served as the Commissioner of Dewa for the Muromachi bakufu. This individual served five generations before the widely known daimyō, Mogami Yoshiaki (written with a different character for “aki”), the eleventh head of the Mogami clan.
The circumstances in Dewa during the era of Yoshiaki are not well-documented and are the subject of continuing research. His year of birth and father are unknown. There are theories that Yoshiaki was either the third son of Mogami Mitsuie (the fourth head of the Mogami clan) or the eldest son of Mogami Yoshiharu (the fifth head of the Mogami clan). If he was the third son of Mitsuie, then Mogami Yorimune and Yoshiharu would have been his older brothers. The character “yoshi” is surmised to have originated from the Ashikaga shōgun family, but it is not certain which shōgun. In 1474, at the height of the Ōnin-Bunmei War, Yoshiaki inherited the headship of the clan following the death in battle of his predecessor, Yoshiharu.
A conflict between Shiba Yoshitoshi (the tenth head of the Shiba clan) and Shiba Yoshikado (the eleventh head of the Shiba clan) was one of the causes of the Ōnin-Bunmei War. As a member of the Shiba clan, the Mogami are believed to have played a role, but there are no traces in historical records. Yoshiaki died six years after inheriting the role as the head of the clan. His younger brothers (or sons), Mogami Yoshifusa and Mogami Yoshigi, were allocated the fiefs of the Narusawa clan and the Sugiyama clan, respectively. Either Yoshifusa or Yoshigi would have been expected to succeed Yoshiaki, but, for certain reasons, Nakano Mitsuuji, the son of Nakano Mitsumoto, inherited the Mogami clan instead, adopting the name of Mogami Mitsuuji. The death of Mitsuie’s eldest son, Mogami Yorimune, is believed to have occurred in connection with a political incident although the circumstances are unknown.