Born in 1491 as the head of the Nagano clan, a powerful local family in Kozuke Province. Based at Minowa Castle (present-day Takasaki, Gunma), he served the Kanto kanrei Uesugi clan. Even after the Uesugi's decline, he maintained his power in western Kozuke. From 1557 onward, when Takeda Shingen launched full-scale invasions into western Kozuke, Narimasa defended Minowa Castle and repelled the Takeda forces repeatedly—tradition says as many as six times—reportedly making Shingen lament: 'As long as Narimasa lives, Kozuke cannot be taken.' He died of illness on November 22, 1561, at age 70. His dying instructions to his heir Narimori were: 'No funeral for me. Put armor in my coffin. If the Takeda come, die fighting with the castle as your pillow.' After Narimasa's death, Shingen renewed his invasion, and Minowa Castle fell in 1566.

Tough and stubborn, yet deeply trusted by his retainers and people. He had the spirit to defend his domain by his own strength without relying on central authority, maintaining his warrior's pride even on his deathbed.

Revered as the model of a Kozuke warrior for repeatedly repelling Takeda Shingen. Minowa Castle is listed among Japan's 100 Fine Castles and continues to attract visitors drawn by Narimasa's fame. He is considered one of the warriors Shingen feared most.