Born in 1521, Ashina Moriuji was the 16th head of the Ashina clan. He succeeded his father Morikiyo as clan head in 1541 and, through a strategic marriage to a daughter of Date Tanemune, built an alliance while expanding his domain from Aizu into northern Echigo, earning renown as the "restorer of the Ashina clan." In 1561 he retired in favor of his heir Morioki but that same year began building a retirement fortress on Mount Iwasaki at the southern edge of the Aizu basin. After eight years of construction, Mukaihaguroyama Castle — one of Japan's largest mountain castles — was completed in 1568, and Moriuji made it his base until his death in 1580. Even after taking Buddhist vows as "Shishisai," he retained real power, forming a de facto dual leadership with Morioki, who held the main castle at Kurokawa.

A strategist who skillfully combined political marriages with military expansion to build the Ashina clan's greatest territorial extent, and who retained real power even after formally retiring.

He is renowned for building Mukaihaguroyama Castle, one of Japan's largest mountain castles, designated a National Historic Site in 2001. After his death, the Ashina clan weakened amid succession disputes and was ultimately defeated by Date Masamune at the Battle of Suriagehara in 1589.