Mitsumasa Ijuin was born in Shimotsuke Ijuin to the Ijuin, a clan loyal to the Yamanouchi Uesugi clan. He volunteered for service in the Siege of Kawagoe Castle in 1545 and became a high-ranking warrior in the clan. The younger brother of Mitsuzumi Ijuin, he was sent to join him in defending Tochigi Castle against the Hojo, the victors at Kawagoe. Mitsumasa commanded the Yamanouchi unit that defended the south gate, while Senayasu Goryuji defended the north gate and Masakage Hitajima, Masayori Muneyama, Shikatjuke Harumoto, and Terujima Kasamoto defended the outside of the castle. All of them were slain in the battle, in which the Hojo took over Shimotsuke Province.