Mogami Yoshiaki was the first son of Mogami Yoshimori (最上 義守), and succeeded his father as daimyō of Yamagata. 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 due to the fact that Hideyoshi ordered the execution of Yoshiaki's teenage daughter when purging his nephew Toyotomi Hidetsugu, whom Yoshiaki's daughter was engaged to.