Born in 1204, eldest son of Miura Yoshimura, he inherited the Miura clan's leadership. Unlike his scheming father, he was known for a mild, honorable disposition. In the shogunate, his close ties to the fourth shogun Fujiwara Yoritsune marked him as the center of anti-Hojo sentiment. When Yoritsune was deposed by regent Hojo Tsunetoki in 1244, pressure on Yasumura and like-minded vassals intensified. In 1247, fifth regent Hojo Tokiyori and Adachi Kagemori conspired to destroy the Miura (the Hoji War). Caught off guard before he could muster adequate forces, Yasumura was driven from Wakamiya-oji to Hokke-do in Kamakura, where he committed suicide with his brother Mitsumura and over 500 clansmen. The Hoji War obliterated the Miura—one of the most powerful vassal houses since the shogunate's founding—and firmly established Hojo tokuso autocracy. He was 44.

Less cunning than his father, he was considered gentle and loyal. But his moderate nature drew Hojo suspicion, leading to his tragic end.

The Miura clan's fall in the Hoji War transformed the shogunate's power structure, establishing Hojo Tokuso autocracy that lasted until the shogunate's end.