Houjou Ujinori was the fifth-born son of Houjou Ujiyasu (fourth son to survive childhood), younger brother of Houjou Ujimasa, Houjou Ujiteru, and Houjou Ujikuni, and older brother of Houjou Ujitada, Houjou Saburou (Uesugi Kagetora), and Houjou Ujimitsu. He was the master of Misaki Castle in Sagami and chamberlain of Nirayama Castle in Izu. He married Kougen'in, the daughter of Houjou Tsunashige.
As a child, Ujinori was sent to Suruga as a hostage of Imagawa Yoshimoto. It was said that he became friends with Tokugawa Ieyasu, who was also a hostage in Suruga at the time, during this period. He returned to the clan somewhere in the period between 1558-1570, and in 1571 was again sent as hostage to the Takeda Clan in Kai along with his younger brother Houjou Ujitada.
Though his brothers Ujiteru and Ujikuni were known for their diplomatic skills, Ujinori surpassed both of them. This finesse was evident in his negotiations with Uesugi Kenshin and Takeda Katsuyori, and later in alliances with Tokugawa Ieyasu, Date Masamune, and the Ashina Clan. Tokugawa Ieyasu saw Ujinori as his window into the Houjou Clan, and communicated with him extensively.
Ujinori journeyed to the capital several times to negotiate with Toyotomi Hideyoshi as his brother Ujimasa's representative, but these negotiations failed, and Hideyoshi attacked Odawara Castle in 1590. Ujinori withstood siege from Hideyoshi's army for 3 months, but finally surrendered on Ieyasu's recommendation. He was also given the role of persuading his brother Ujimasa to surrender.
After the battle, Ujinori followed Houjou Ujinao to Mt. Kouya. He was pardoned in 1591 by Hideyoshi and given a territory of 2000-koku, then 6,980-koku in Kawachi and Sayama Castle in 1594. He died of illness at the age of 60, and his son Houjou Ujimori was allowed the continued governance of Sayama-han. His line continued until the Meiji Restoration.