Mogami Yoshiaki was the first son of Mogami Yoshimori (最上 義守), of the Mogami clan and succeeded his father as daimyō of Yamagata. When Yoshihime, who was his sister, married Date Terumune, the Mogami clan allies with the Date clan. During this time, Yoshihime wrote information about the Date clan to Yoshiaki, her and Yoshiaki's actions made the Date clan distrust the Mogami family. Yoshiaki fought against the Date clan twice in different years, in both battles Yoshihime advanced to the middle of the battlefield to create a peace treaty.
When Toyotomi Hideyoshi came to power, Mogami submitted to his rule, but later became a supporter of Tokugawa Ieyasu following Hideyoshi's death. He was known to hate the Toyotomi because Hideyoshi ordered the execution of Yoshiaki's teenage daughter when purging his nephew Toyotomi Hidetsugu, whom Yoshiaki's daughter was engaged to.
In 1600, he battled Uesugi Kagekatsu, an enemy of Tokugawa's, alongside Date Masamune (his nephew), another lord of the far north. He aided in Date's siege of Shiroishi, and was then attacked in his own home castle of Hataya and Hasedō, Later that year, Mogami and Date supported Ieyasu at the famous battle of Sekigahara, after which Mogami's domain was expanded to 520,000 koku in return for his loyal service. This made the Yamagata domain the fifth largest in Japan at the time, excluding the land held by Tokugawa.
He died at Yamagata Castle in 1614. Yamagata maintains the Mogami Yoshiaki Historical Museum, just outside the rebuilt Great Eastern Gate of Yamagata Castle, which displays his helmet, battle command baton and other implements he actually used.
Mogami Yoshiaki laid out and built the castle town, which became the foundation of modern-day Yamagata City. He controlled the "Three Difficult Places" on the Mogami River, making navigation safer from the Sea of Japan to the inland, and bringing the culture of Kyōto and Ōsaka to Yamagata. His dam building projects at Kitadaseki, Inabazeki and other places, and other irrigation control measures helped develop rice cultivation in the Shōnai plain.
| First Daimyō of Yamagata