Yoshimoto Imagawa was born in 1519 in Suruga Province of Japan, and was in control of a very powerful clan by the time he was its head in 1545. Imagawa was one of the three warlords in Kanto Province, a participant in the Kanto Wars. In 1545, he aided the Takeda clan and the Uesugi clan against the Hojo clan, led by Ujiyasu Hojo. Yoshimoto was the victor of the Sagami-Suruga Conflict of 1558 where the Imagawa and Takeda repelled an invasion by the Hojo and the Uesugi. Alliances shifted during the war, with Yoshimoto fighting alongside his brother-in-law Shingen Takeda sometimes, but also crossed blades with him frequently. In the end of the conflict, no clans had made any progress.
Imagawa also fought against the Oda clan in the 1540s, fighting against Nobuhide Oda and his powerful clan. By 1545, he was fighting a huge war against the Oda, and was supported by the Tokugawa clan. The Imagawa clan was successful in their war against the Oda, and by 1560, they were prepared to launch a campaign on Kyoto to overthrow the Ashikaga Shogunate and restore the Imagawa power. Yoshimoto Imagawa, in command of the 25,000-strong army, led his forces in a clash at Okehazama with the young commander Nobunaga Oda, but suffered a rare reverse; Nobunaga stormed his camp and killed his general Naomori Ii before a soldier, sometimes identified as Katsuie Shibata, headed into Yoshimoto's tent and beheaded him. Shortly after, the weak Imagawa peonic army fled the field, and the Battle of Okehazama marked the end of the Imagawa's strength.