Yoritomo MINAMOTO raised an army and Yasutoki's father, Yoshitoki, along with the Hojo family (including Yasutoki's grandfather Tokimasa HOJO) followed Yoritomo, whereupon they entered Kamakura; in or around the fourth year after this entrance, Yasutoki was born when his father, Yoshitoki, was 21 years old.
父義時は21歳、祖父北条時政ら北条氏一族と共に源頼朝の挙兵に従い鎌倉入りして4年目の頃である。