Amago Kiyosada served as the third head of the Amago clan, a branch of the Sasaki and cadet family of the Kyōgoku clan. Kiyosada was the deputy military governor, or shugodai, of Izumo Province and lord of Gassantoda Castle, a mountain fortress in Izumo.  He was the son of Amago Mochihisa.

Kiyosada received one of the characters in his name from Kyōgoku Mochikiyo. Mochikiyo served as a shugo daimyō, including military governor and head of the samurai-dokoro, or local police function of the Muromachi bakufu in Yamashiro Province, in addition to military governors of the provinces of Izumo, Oki, Hida, and Ōmi.

Kiyosada inherited the role of deputy military governor of Izumo from his father around 1467.  After the outbreak of the Ōnin-Bunmei War, areas under control of the Kyōgoku clan who were the military governors of Izumo rapidly deteriorated, fueling aspirations by local clans of influence opposed to the Kyōgoku, including the Matsuda and Mizawa clans.  Kiyosada suppressed their actions and stopped an invasion of Izumo by the Yamana.  Mochikiyo recognized these contributions by awarding Kiyosada the post of bugyō, or magistrate, in the Nogi District and as the official in charge of taxation on behalf of the bakufu in Mihonoseki.  From 1469 to 1471, Kiyosada engaged in conflicts in several locations in Izumo such as the Ōhara District, in Mihonoseki, and at Inanba Castle.  

Kiyosasa strengthened his position in eastern Izumo, and, showing signs of independence, from 1474, was delinquent in paying taxes to the central authorities.  In 1476, he suppressed an uprising among local peasants in the Nogi District.  In 1479, his eldest son, Tsunehisa, pacified neighborhoods in the Nogi and Ou districts of Izumo.