In 1183, Mitsunaga entered Kyoto as part of the army of Yoshinaka KISO who advanced from Hokurikudo to Kyoto and was assigned to Hoki no kami (the Governor of Hoki Province) in an appointing ceremony in August.
寿永2年(1183年)、北陸道より進軍した木曾義仲に従い入京し、8月の除目で伯耆守に任じられる。