Kazutoyo Yamanouchi was the son of Moritoyo Yamanouchi, a warlord from Honshu. From 1565 to 1582 Kazutoyo served as a commander for Nobunaga Oda. After his death, he became a vassal of the Toyotomi clan under Hideyoshi Hashiba, and became an independent daimyo in the power vacuum that followed his death in 1598. He took part in a coalition led by Masanori Fukushima against Mitsunari Ishida, a former Toyotomi strategist who had hoped to continue the Toyotomi legacy rather than aid the Tokugawa, who were seen as the Toyotomi clan's successors. After the coalition was defeated Kazutoyo and Masanori joined Tokugawa's Eastern Army and fought at the Battle of Sekigahara, and after the victory there, he was entitled to the Tosa Domain, a clan far from his home, in the territory of the former Chosokabe clan. He ruled for only five years, when he died.