[3] Mogami Yoshiaki was born on the first day of the first month of the Tenbun era (1546) to his father, Yoshimori, [2][1] and his mother, Nagaura.

[2] On June 14, 1563 (Eiroku 6), Yoshiaki, aged 17, and his father went to Kyoto to have an audience with Shogun Yoshiteru and present him with a horse and a sword.

Yoshiaki, who inherited the family headship, used fairly forceful methods to suppress their attempts to declare independence from the Mogami clan.

Yoshiaki fought against the Date clan twice in different years, 1574 and 1578, in both battles Yoshihime advanced to the middle of the battlefield to create a peace treaty.

It is recorded that when Shiratori Nagahisa was about to receive the family tree document, Yoshiaki suddenly killed him with a sword that he had hidden.

Yoshiaki's eldest son, Mogami Yoshiyasu and Tateoka Mitsushige succeeded in defeating the Onodera reinforcements.

[citation needed] In October, Yoshiaki crossed the Rokuji Highway and invaded Shōnai in an effort to reinforce Tōzenji Yoshinaga, lord of Tozenji Castle, against the attack of his enemy, Daihōji Yoshioki.

This resulted in Hidetsugu being accused of treason and forced to commit seppuku at Mount Kōya on the order of Hideyoshi, presumably to secure the inheritance of Hideyori.

Furthermore, Yoshiaki was suspected of participating in Hidetsugu's rebellion, but he was pardoned with the assistance of Tokugawa Ieyasu's intercession on his behalf.

As result of the execution of Komahime, Yoshiaki's wife Osaki-dono was struck with deep grief by the sudden death of her daughter, and she died on August 16.

Mogami faced a dilemma because he still supported Ieyasu, but if he did not join the Toyotomi side, there was a high possibility that he would be attacked by the Uesugi.

As Yoshiaki believed in Ieyasu's victory in the Kamigata region, he tried to buy time, but in the end, the Uesugi forces invaded the Mogami territory on September 8.

However, it was concluded that the guerilla warfares conducted by Yoshiaki to harass the route of the Uesugi army played role in the successful defense of the castle.

[2] It was said that at one occasion, a group of Mogami clan's ninja infiltrated the camp of Naoe Kanetsugu, stealing his battle standard, which later hoisted on the Hasedō Castle's gate, which greatly demoralized the Uesugi troops.

In the end, when information came in that Tokugawa Ieyasu emerged victorious in the main Sekigahara battle, The uesugi forces under Kanetsugu began to withdraw from Hasedō Castle.

Switching positions, Yoshiaki began to pursue them, but Kanetsugu himself concentrated his riflemen to defend his retreating army, and managed to withdraw safely back to Yonezawa.

[18] After the campaign ended, the Mogami clan was rewarded with an increase in size from 200,000 to 570,000 koku for their domain in return for their loyalty to the Eastern army during the war.

[citation needed] On May 11, Ieyasu, who had established his hegemony over the Toyotomi government, visited the Imperial Palace to receive an audience with the Emperor.

He was accompanied by Mogami Yoshiaki as well as Mori Tadamasa, who had been instrumental in the attack on the Sanada clan, and Ii Naomasa, who had led the vanguard in the Battle of Sekigahara, and Oda Nagamasu.

He also constructed a massive irrigation system (called Kitadate Ozeki) in Shonai, with a total length of over 30 kilometers, and created 4,200 hectares of vast rice fields.

He followed the advice of his vassal Kitadate Toshinaga, to override opposition from those around him and give permission for the construction projects, providing financial and material support to ensure their success.

[2] However, after this, the relationship between Yoshiaki and his eldest son, Yoshiyasu, deteriorated due to a power struggle between their respective vassals.

As the conflict continued, Yoshiaki worried that this could led to a civil war which would potentially cause the shogunate to strip the Mogami clan's control from their regions.

Yoshiaki thought that if he allowed Iechika to succeed him as head of the family, he would be able to avoid the Mogami clan being punished by the shogun.

He controlled the "Three Difficult Places" on the Mogami River, making navigation safer from the Sea of Japan 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.