[1] The Nagao clan descend from military lord Taira no Yoshifumi, of the Kammu Heishi (Taira clan), and from the Emperor Kammu (735–806), the 50th Emperor of Japan.

They are one of the 'Bando Hachi Heishi', the 'eight Taira clans of Kanto region' (the Chiba, Miura, Nagao, Kazusa, Doi, Chichibu, Oba, and Kajiwara clans, respectively).

The Nagao were the Kasai (Head retainers) of the Uesugi clan, and were the Shugodai (vice-Governors) of Echigo, Kozuke, and Musashi provinces.

The Shirai Nagao branch were Shugodai (vice-Governors) of Kozuke and Musashi provinces and lords of Aomi, Hachigata and Shirai castles.

This branch of the clan built and controlled Kasugayama Castle and the surrounding fief, in what is now Niigata Prefecture.