Born as the eldest son of Rusu Masakage, lord of Toshiro Castle in Miyagi and uncle of Date Masamune. Masakage was permitted by Masamune to take the Date surname after his service at Sekigahara, founding the Rusu line of the Date family. Muneyoshi inherited the headship in 1607 and became lord of Ichinoseki, then in 1629 was granted transfer to Mizusawa Castle in recognition of his work on Edo Castle's stone ramparts. He went on to develop the castle town and open new farmland, and the Mizusawa-Date house he founded ruled the area for 240 years until the end of the Edo period.

A steady, practical builder and town planner. From a peripheral position within the Date clan, he steadily accumulated administrative achievements and earned his lord's trust, culminating in his transfer to Mizusawa.

He founded the 240-year Mizusawa-Date line. The castle and castle town he developed became the foundation of the streetscape of present-day Mizusawa, Ōshū City, sometimes called "the little Kyoto of Michinoku."