Nagachika became a vassal of Uesugi Kenshin when Kenshin went to the capital in 1559 and recognized Nagachika's ability. Nagachika, who was both courageous and wise, served as a magistrate for Kenshin. He fought in many of Kenshin's battles, including the battles for the Kantou against Houjou Ujiyasu. He served as chamberlain of Numata Castle, then master of Uozu Castle and supreme commander of Uesugi's troops in Ecchuu. He succeeded Uesugi Kagenobu as head of the Koshi-Nagao family, but refused the Nagao family name.
He became a monk after Kenshin's death. At the beginning of the Otate no Ran, he was neutral towards Uesugi Kagenobu and the Kawada family's support of Uesugi Kagetora, but later supported Uesugi Kagekatsu.
He fought a defensive battle against Oda's invading army, and later met the invasion of Ecchuu by Shibata Katsuie, Sassa Narimasa, etc. but died of illness during the campaign. Invigorated by his death, the Oda army took over the entire province of Ecchuu.