Nagao Masanaga served as a bushō during the Sengoku period.  He served as the head of the Ashikaga-Nagao clan.

In 1527, Masanaga was born as the son of Nagao Norinaga.  Masanaga inherited the headship of the clan from his father.  He received one of the characters in his name from Uesugi Norimasa, adopting the name of Masanaga.  He served as the kasai, or head of house affairs, for the deputy shōgun family of the Kantō.  In 1548, owing to his efforts for the coming-of-age ceremony for Ashikaga Fujiuji, he received the honorary title of Governor of Tajima.  However, after Norimasa withdrew from the Kantō owing to suppression by the Gohōjō clan, the role of the head of house affairs for the deputy shōgun family of the Kantō was extinguished and Masanaga also surrendered to Hōjō Ujiyasu.

In 1560, Masanaga entered the priesthood, adopting the monk’s name of Zenshō, but, in the eleventh month of the same year, he returned to secular life and changed his name back to Masanaga.  In the third month of 1561, after Uesugi Norimasa transferred the headship of the clan and the role of the deputy shōgun of the Kantō to his adopted son, Nagao Kagetora (later known as Uesugi Kenshin), in the fourth month, Masanaga received one of the characters in his name from Kagetora.  Similar to his grandfather, he adopted the name of Kagenaga.  Before long, Kagetora (who changed his own name to Uesugi Masatora) initiated deployments in the Kantō.  As a member of the same Nagao clan, Kagenaga supported him.  Masatora proceeded to suppress the Gohōjō clan.  Later, as a means to oppose Takeda Shingen of Kai Province, an alliance was formed with the Hōjō known as the Etsu-Sō dōmei, or the alliance between Echigo and Sagami provinces.  Thereafter, Masanaga engaged in negotiations between the Uesugi and Hōjō clans in the diplomatic and military spheres.

In 1569, after the demise of Kagenaga, Masanaga was succeeded by his son-in-law, Nagao Akinaga.  His base at Tatebayashi Castle, however, was granted to another son-in-law named Hirota Naoshige.  Another one of Kagenaga’s sons received one of the characters in his name from Hōjō Ujimasa, adopting the name of Nagao Masanaga (written with a different character than his father’s earlier name), but given that Akinaga inherited the clan, it is surmised that this son died at an early age.