Morimasa was born in what is now Shōwa-ku, Nagoya (situated in contemporary Aichi District, Owari Province), He was a retainer of Shibata Katsuie and one of his top generals in many of his campaigns.

Later, Morimasa headed for a rescue in response to a demand by Maeda Toshiie and beat the Uesugi army stationed at the back of the Arayama Castle at Noto Province.

However, he ignored Shibata Katsuie's orders to fall back and this led to his defeat as Toyotomi Hideyoshi's forces approached the next morning.

[1] The charge by Morimasa was the spark necessary for the battle of Shizugatake where Hideyoshi's troops were able to suppress any resistance led by Maeda Toshiie and prevented the support of Sassa Narimasa and Takigawa Kazumasu.

This eventually forced Shibata Katsuie to commit seppuku along with his wife Lady Oichi (younger sister of Nobunaga) following the betrayal of Maeda Toshiie.