Born the eldest son of Mogami Yoshimori, lord of Yamagata Castle in Dewa Province (Yamagata). The Mogami were a distinguished family branched from the Shiba, a line of the Ashikaga clan. His younger sister Yoshihime later married Date Terumune and became the mother of Date Masamune, tying the Mogami and Date houses closely together.
Around this time his father Yoshimori is said to have handed him the headship (the date is disputed). But around the Mogami were many lords, such as the Tendō and Sagae, who were kin yet held independent power, so the head of the house was weak. Yoshiaki first had to bring his own house together.
He clashed with his father Yoshimori and fighting broke out (the Tenshō Mogami Disturbance). His father was backed by his son-in-law Date Terumune, but Yoshiaki held out, and in time they were reconciled and his leadership of the Mogami was secured. This conflict also began his complicated relationship with his nephew Date Masamune.
He attacked and destroyed the Tendō, brought the Sagae and others under his control, and largely united the Murayama region around the Yamagata basin. Yoshiaki relied not only on battle but also on schemes to win over opponents, and is said at times to have used harsh means. In this way he grew into a leading daimyō of Dewa.
He fought the Uesugi side's Honjō Shigenaga and others over the Shōnai region on the Sea of Japan coast, but was defeated at the Battle of Jūgorigahara and lost Shōnai. Shōnai was rich land, and Yoshiaki long hoped to win it back, a wish fulfilled after the Battle of Sekigahara.
During Toyotomi Hideyoshi's Odawara campaign he went to Hideyoshi, submitted, and had his lands confirmed. From around this time he also grew close to Tokugawa Ieyasu, sending his son Iechika to serve Ieyasu. By tying himself to the great powers at the center, he sought to ensure the survival of the Mogami house.
His daughter Komahime had just gone up to Kyoto to marry the regent Toyotomi Hidetsugu when Hidetsugu was accused and forced to take his own life; caught up in the affair, she was put to death along with Hidetsugu's wives and children. Soon after, Yoshiaki's wife also died, and it is said that in his deep grief his feelings toward the Toyotomi changed.
At the time of Sekigahara, Yoshiaki sided with the Tokugawa and fought the army of Uesugi Kagekatsu of Aizu. A large Uesugi force led by Naoe Kanetsugu fiercely attacked Hasedō Castle, but the castle held, and when news of the Western Army's defeat at Sekigahara arrived, the Uesugi withdrew. This fighting is also called the "Sekigahara of the North."
For his services he was granted the long-desired Shōnai and other lands, becoming a great daimyō of about 570,000 koku. Yoshiaki greatly enlarged Yamagata Castle and laid out its castle town, and is said to have worked on boat transport along the Mogami River and on irrigation canals. The foundations of today's Yamagata City were laid at this time.
Though a warlord, Yoshiaki was deeply fond of renga linked verse throughout his life; records show he associated with the Kyoto renga master Satomura Jōha and others and took part in many renga gatherings. In a life full of war, he also had the side of a cultivated man who loved waka and renga.
He died of illness at Yamagata Castle. In his last years the house was shaken by disputes over the succession, and he suffered sorrows such as the loss of his eldest son Yoshiyasu. Internal strife continued after his death, and in 1622 the shogunate confiscated the Mogami lands, but his achievements in building Yamagata are still remembered.
Toyotomi HideyoshiTokugawa IeyasuUesugi KagekatsuNaoe KanetsuguDate Masamune
Takenaka Shigeharu (Hanbei)Azai NagamasaKuroda Yoshitaka (Kanbei)Takeda KatsuyoriOichiSanada MasayukiAsano NagamasaSatake Yoshishige
People of the Sengoku/Azuchi-Momoyama period ・ People by period
Based on facts found in textbooks and biographies. Uncertain points are marked as such.