Born in 1162, Munemasa was the second son of Oyama Masamitsu, a powerful clan leader in Shimotsuke Province. In 1184, he built Naganuma Castle in the Naganuma area of Haga District, Shimotsuke (present-day Naganuma, Moka City, Tochigi), and took the Naganuma surname from the place. After pledging allegiance to Minamoto no Yoritomo in 1180, he fought at the Battle of Nogi-no-miya in 1183, defeating Shida Yoshihiro, and campaigned in the Oshu Expedition of 1189. Following the Jokyu War of 1221, his military service earned him appointment as shugo of Settsu and Awaji Provinces; the Awaji shugo post was passed down within the Naganuma family until the end of the Kamakura period. The Azuma Kagami chronicle describes him as a man of "wild words and abuse," a hot-tempered Bando warrior. He died in 1240 at age 79 in the Naganuma estate of Shimotsuke.

Fiery and outspoken to the point that the Azuma Kagami singled him out for his "wild words and abuse," he was unafraid to speak bluntly even to shoguns and regents — a quintessential Bando warrior.

The castle he built remained the seat of the Naganuma clan for generations, and the shugo posts he earned in Settsu and Awaji were held by his descendants for years, establishing the Naganuma as an enduring Kamakura-period gokenin house.