Born in 1200, second son of the second shogun Minamoto no Yoriie. As a young child he was ordained at Tsurugaoka Hachimangu, entering the monastery through his mother's connections. His resentment grew year by year as his father Yoriie was killed by Hojo machinations at Shuzenji in 1204, and the shogunate passed to his uncle Sanetomo. Kugyo maintained contact with Sanetomo and shogunal elders while quietly training in swordsmanship and awaiting his moment. On January 27, 1219, when Sanetomo came to Tsurugaoka Hachimangu to celebrate his appointment as Grand Councillor, Kugyo hid behind the great ginkgo tree. As Sanetomo descended the stone steps after the ceremony, Kugyo leapt out shouting "Thus I avenge my father!" and cut him down. He fled with the head but was quickly tracked down and killed by Nagao Sadakage, a retainer of Miura Yoshimura. He was 20. The assassination extinguished the Minamoto shogunal line and marked the turning point at which Hojo regency rule fully began. Persistent theories hold that Miura Yoshimura or the Hojo had a hand in the plot.

A revenge-driven monk consumed by the desire to avenge his father. Despite his clerical status, he trained in combat and carried out the assassination. Some suspect Miura or Hojo conspiracy behind the act.

Sanetomo's assassination was a turning point in Japanese history. The end of Minamoto shoguns led to full Hojo regency. The great ginkgo tree at Tsurugaoka was famously said to have hidden Kugyo.