Mibu Tsunashige served as a bushō during the late Muromachi and Sengoku periods.  He was a retainer of the Shimotsuke-Utsunomiya clan and served as the lord of Mibu and Kanuma castles in Shimotsuke Province.  Tsunashige was the second head of the Mibu clan.

In 1448, Tsunashige was born as the son of Mibu Tanenari, a bushi and retainer of the Shimotsuke-Utsunomiya.

Upon orders of his lord, Utsunomiya Tadatsuna, Tsunashige attacked the Kanuma clan and was assigned governance of Kanuma, establishing his base at Kanuma Castle.  According to the diary of a renga master named Sōchō, however, in 1509, the Mibu had already advanced to the area of Kanuma.  Until the time of Tsunashige’s demise, Tadatsuna’s father, Utsunomiya Shigetsuna, continued to maintain his grip on power despite being retired so it is certainly possible that Tsunashige invaded Kanuma upon orders of Shigetsuna.  Moreover, based on accounts of the Minagawa, the Mibu governed Kanuma beginning in 1523 after the Battle of Kawarada.  This was after the time of Tsunashige’s death so it is uncertain which account is correct.

He assigned his lineal heir, Mibu Tsunafusa, to Mibu Castle.

In 1509, when Sōchō came to Shimotsuke to see the check point in Shirakawa (a location along transit routes to collect taxes and inspect travelers), Tsunashige invited him to his residence in Kanuma to a hall in Kanuma to hold a renga event.  At this time, Tsunashige and Sōchō (who were of the same age) cultivated friendly relations and enjoyed composing renga together.  In 1512, an internal conflict erupted between Utsunomiya Shigetsuna and Haga Takakatsu known as the Utsunomiya Disturbance.  During this conflict, Tsunashige cooperated with Shigetsuna, contributing to effort to calm the situation.  Consequently, he earned the position as a senior retainer of the Utsunomiya clan.  Thereafter, he gradually expanded his authority and garnered control of the area of Nikkōsan.

On 9/17 of Eishō 13 (1516), Tsunashige died at the age of sixty-nine.