Houjou Ujinao was the eldest son and heir of Houjou Ujimasa. His mother was Ujimasa's principle wife, Oubaiin, daughter of Takeda Shingen. He became the fifth head of the Later Houjou Clan in 1580 when his father retired. He married Tokuhime, daughter of Tokugawa Ieyasu, in 1583 after a year of battles between the Houjou and Tokugawa armies over the provinces of Kouzuke, Shinano, and Kai, left masterless after the deaths of both Takeda Katsuyori and Oda Nobunaga.

After the Siege of Odawara in 1590, Toyotomi Hideyoshi spared Ujinao's life in part because he was Ieyasu's son-in-law. He was exiled to Mt. Kouya along with his brothers, uncles, and retainers. There he lived under house arrest until the beginning of 1591, when Toyotomi granted him a fief of 10,000-koku in Osaka and allowed his wife to join him there. However, he died there in November of the same year of smallpox at the age of 30.