A daimyo of Dewa Province. Born the eldest son of Mogami Yoshimori, he suppressed the internal clan strife (the Mogami clan disturbance) that had persisted since his father's era, firmly establishing his position as head of the Mogami. His sister Yoshihime's marriage to Date Terumune made him the uncle of Date Masamune, yet they fiercely competed for supremacy in Dewa, with near-direct conflict arising in 1588. Adroitly alternating between political strategy and force, he became the dominant daimyo over most of Dewa. At the 1600 Keicho Dewa Campaign—a prelude to Sekigahara—he held castle Hasedo against the Uesugi army led by Naoe Kanetsugu, the general of Uesugi Kagekatsu. When the Eastern army won at Sekigahara the Uesugi were forced to withdraw, and for his service Yoshiaki was rewarded with an expanded domain of 570,000 koku in Dewa-Yamagata, becoming one of the most powerful daimyo in the Tohoku region. However, after his death the Mogami were dispossessed in 1622 due to the internal succession dispute known as the Mogami Disturbance, and the clan vanished from history's main stage. His was the life of a strategist called 'the Fox of Dewa.'

A calculating and bold warlord who did not hesitate to purge even family members in his drive for power. Yet he was also versed in tea ceremony and poetry—more than just a warrior.

Under Yoshiaki's rule, Yamagata flourished as the castle town of a 570,000-koku domain. He developed Yamagata Castle (Kajo) and opened the Mogami River waterway. His equestrian statue stands in Kajo Park, Yamagata City. Though the Mogami were later dispossessed, Yoshiaki's own achievements are highly regarded.