Akiage Munenobu served as a retainer for the Amago clan of Izumo Province. Munenobu was one of ten elite soldiers referred to after the Sengoku period as the Amago jūyūshi, or the Ten Warriors of the Amago. These soldiers served in a coordinated effort to restore the Amago clan to power in Izumo after losing the province in 1566 following an invasion by Mōri Motonari. The plan was devised by Amago Katsuhisa, together with Yamanaka Yukimori and Tachihara Hisatsuna, in 1568. Yukimori served as the leader of the Amago revival army to battle against the Mōri in a prolonged effort to reclaim their territory in Izumo and neighboring provinces. Prior to the ousting of the Amago, Munenobu is cited in historical accounts as having a role in helping defend Yukimori during a duel known as the One-on-One Duel between Yamanaka Yukimori and Shinagawa Masakazu.
The Akiage clan held the status of shake, or proprietors of a designated shrine. The clan managed a shrine known as the Ōba-Ōmiya (later known as the Kamosu Shrine) in Izumo. Munenobu was the second son of Akiage Tsunahira. His older brother, Akiage Takashige, inherited the role of proprietor of the shrine, so Munenobu served as a retainer of the Amago. As the lead bushō for his family, his stipend was a fief of 10,000 koku in Bingo Province.
In 1563, Munenobu joined his father, Tsunahira, as reinforcements in opposition to an attack by the Mōri against Shiraga Castle in Izumo, but suffered a major loss. After the fall of Shiraga, Munenobu and Tachihara Hisatsuna attempted an assault against Sugihara Morishige at Odaka Castle in Hōki Province, once again incurring a loss. In 1565, Munenobu supported the defenders against a major assault by the Mōri clan against Gassantoda Castle, defeating an army led by Kikkawa Motoharu. In 1566, Gassantoda fell to the Mōri and the Amago clan was decimated. Munenobu was captured by the Mōri but released out of respect for the status of the Akiage as proprietors of a shrine.
In 1569, Yamanaka Yukimori persuaded Amago Katsuhisa to launch a plan to revive the Amago family by raising arms at Chūyama Castle in Izumo. Munenobu and Tsunahira joined the campaign. Munenobu was appointed as the commanding general for an assault against the Mōri at Gassantoda Castle, but was deceived by Amano Takashige in a letter of surrender that, in fact, was a set-up for a surprise attack against his forces on their approach to the castle. His forces scattered and the attack on Gassantoda Castle stalled. In the first month of 1570, the Mōri attacked Takuwa Castle en route to confronting the Amago revival army. Prior to their arrival, Munenobu lost the will to fight and attempted to retreat, but suffered major losses in the ensuing pursuit by the Mōri. The following month, Munenobu, along with Yokomichi Hidetsuna, fought and lost in the Battle of Fubeyama. Hidetsuna died in the battle while Munenobu fled.
In the third month of 1571, Munenobu joined Hakura Magobei to attack Yonago and Odaka castles, but Magobei was killed in action and the contingent failed to capture either castle. Months later, Nomura Shietsu, a former retainer of the Amago who had switched allegiance to the Mōri, finally persuaded Munenobu and Tsunahira to surrender to the Mōri. Based on one theory, Munenobu surrendered because he could not fulfill his promise to Amago Katsuhisa to achieve victory after he and Yamanaka Yukimori were entrusted with critical roles in the restoration plan. According to one theory, a bushō named Hino Yukifusa who served Kamei Korenori actually referred to Munenobu. In that case, on 11/29 of 1594, he died of illness in Aki Province.