Houjou Ujimitsu was the eighth (maybe ninth)-born son of Houjou Ujiyasu (or possibly his younger brother Houjou Ujitaka), seventh son to survive childhood, younger brother of Houjou Ujimasa, Houjou Ujiteru, Houjou Ujikuni, Houjou Ujinori, and Houjou Saburou (Uesugi Kagetora). His wife was the daughter of Houjou Genan, (formerly wife of Saburou), and he was adopted by Genan as inheritor to the Musashi-Kodukue territory.

Ujimitsu served in the border defense between Suruga and Sagami. After the Siege of Odawara in 1590, he was exiled along with the rest of the Houjou to Mt. Kouya, and died there under house arrest.

His son Houjou Ujinori became a direct vassal to Tokugawa Ieyasu.