Narimori became head of his clan at 17 after the death of his father Nagano Narimasa's death in 1561 and his older brother Yoshinari death during the Siege of Kawagoe Castle in 1564. Like his father, he was called a an exceedingly brave warrior.

His father managed to repel several invasions from the Takeda Clan, but after his death Takeda Shingen gathered 20,000 troops for a further assault in 1566. Narimori fought a defensive battle from his main castle of Minowa-jou, but was unable to hold out. He committed suicide along with his entire clan.