Mangata Uemon-no-suke served as a bushi during the Sengoku and Azuchi-Momoyama periods. He served as the lord of Mangata Castle in Hyūga Province.
The Mangata were an illegitimate branch of the Kitahara clan. The Kitahara (in turn, an illegitimate branch of the Kimotsuki clan of Ōsumi Province) rose to become sengoku daimyō in Hyūga Province. Uemon-no-suke was a descendant of a cadet family founded by Mangata Matakurō, the younger brother of Kitahara Hisakane, the sixth head of the Kitahara clan. He was the lord of the manor of Mangata at the northwest end of Masakiin in Hyūga.
In 1558, Kitahara Kanemori, the thirteenth head of the Kitahara clan, suddenly died, and then his daughter who was supposed to wed the lineal heir of Kitahara Kanetaka, also died. This triggered a succession struggle. Kanemori’s formal wife, Asō, was the daughter of Itō Yoshisuke, so the Hyūga-Itō clan intervened and plotted to have Asō re-marry with Uemon-no-suke so the Kitahara family would not inherit the headship of the clan.
In 1559, Yoshisuke purged those who voiced opposition to this plan and pushed-through the engagement of Uemon-no-suke and Asō. After Yoshisuke effectively took-over the Kitahara family, he positioned Uemon-no-suke in the main base of the Kitahara at Mitsuyama Castle. A member of the opposition faction, Shirasaka Shimōsa-no-suke, made plans to revive the Kitahara and, with the support of Shimazu Takahisa, Hongō Tokihisa, and Sagara Yoshihi, reclaimed the former territory of the Kitahara and expelled Uemon-no-suke from Mitsuyama Castle. Nevertheless, following the resuscitation of the Kitahara, members absconded one after another and Mitsuyama Castle fell again to the Hyūga-Itō clan. According to historical accounts of Hyūga, a former retainer of the Kitahara named Taira Hikojūrō (Taira Kanekata) became the next lord of Mitsuyama Castle so it appears that Uemon-no-suke did not return to the castle.
In 1572, in the wake of the Battle of Kizakibaru, the Hyūga-Itō deteriorated and, in the twelfth month of 1577, Yoshisuke temporarily retreated from Hyūga for the protection of the Ōtomo clan of Bungo Province. Uemon-no-suke accompanied him but, thereafter, his whereabouts are unknown.