In 1572, Kazumasu along with Sakuma Nobumori was sent by Oda Nobunaga to provide reinforcement to Tokugawa Ieyasu when he was attacked by Takeda Shingen at Battle of Mikatagahara.

Aside from serving in many of Nobunaga's battles, Kazumasu also rendered service to the Oda through domestic affairs, assisting in the construction of Azuchi castle in 1578 and in land surveys with Akechi Mitsuhide in 1580 in the Yamato province.

Later, Kazumasu was appointed Kantō-kanrei (Shōgun's Deputy in the East) by Nobunaga; in this post, with a portion of Kōzuke Province as his domain, he was assigned to keep an eye on the powerful Hōjō clan, based at Odawara.

Following Nobunaga's death in 1582, Kazumasu defended Oda territory, but lost in the Battle of Kanagawa (1582) against Hojo clan and back to Ise Province.

[6] When he performed badly for killing his cousin, Maeda Tanetoshi, in this campaign, he shaved his head, become a Buddhist monk[7] and retired from battle in shame.