Amago Ujihisa was a family member of the Amago clan and the eldest son of Amago Sanehisa. Alternatively, Ujiyasu may have been the last child of Amago Haruhisa, a shugo daimyō and lord of the clan in Izumo Province.
Although Ujihisa came from a commoner’s family lineage, he was positioned to succeed his grandfather, Amago Kunihisa as head of the shingūtō, the elite fighting force and protectors of the Amago clan. Kunihisa, however, gradually began to favor his son, Amago Takahisa, and aimed to have Takahisa become his successor. This caused Ujihisa to appeal to Haruhisa for support. Despite their role as guardians of the clan, the arrogance and meddling by its leaders caused Haruhisa to view the shingūtō as a threat to his authority for a long time. In 1554, he took action to purge the shingūtō, including Kunihisa and his son, Sanehisa, to reimpose control over the clan. Kunihisa was assassinated while departing the castle, while Sanehisa killed himself in the shingūtō quarters. Remaining members either killed themselves or fled, marking the end of the elite force. Ujihisa survived. At the time of the purge, Tanaka Sanrō-saemon, a retainer of the Amago, feared for the future of the Amago clan, so he spirited away a young Ujiyasu to have him raised by Hachisuka Masakatsu.
In 1566, attacks by the Mōri decimated the Amago clan. In 1569, the fifth son of Sanehisa, Amago Katsuhisa, secured the support of retainers – Yamanaka Yukimori and Tachibana Hisatsuna – to raise an army with the objective to restore the Amago to power. Ujihisa supported the plan for this army to enter Izumo. In 1571, however, their base at Shinyama Castle fell to the Mōri so the army retreated from Izumo. Thereafter, plans were made with the aid of Yamana Toyokuni to invade Izumo from Inaba Province, whereupon Yukimori led Ujiyasu and the army in a clash against Takeda Takanobu. In 1576, the Mōri attacked the army at Wakasa-oniga Castle in Inaba, causing the defenders to retreat.
In 1577, Hashiba Hideyoshi carried out orders from Oda Nobunaga by launching a campaign against the western provinces. The Amago served in the vanguard by attacking Kōzuki Castle in Harima Province. In 1578, however, a deteriorating conditions on the ground caused Hideyoshi’s forces to retreat, isolating the Amago. At the Battle of Kōzuki Castle against the Mōri army, Ujihisa and Katsuhisa turned over the castle and killed themselves, marking the end of the Amago clan.