Mibu Tsunafusa served as a bushō and sengoku daimyō from the late Muromachi to Sengoku periods.  He was a retainer of the Shimotsuke-Utsunomiya clan and served as the third head of the Mibu clan.  Through trickery and guile, Tsunafusa orchestrated the downfall of other senior retainers such as Haga Takatsune and acquired tremendous power within the Utsunomiya clan.  In his later years, he achieved independence from the Utsunomiya, becoming a sengoku daimyō.

In 1479, Tsunafusa was born as the lineal heir of Mibu Tsunashige, a chief retainer of the Shimotsuke-Utsunomiya.  He received one of the characters in his name from his lord, Utsunomiya Shigetsuna.

After his father, Tsunashige, became the lord of Kanuma Castle, Tsunafusa became the lord of Mibu Castle in Shimotsuke.  In 1509, during a visit to Kanuma by a renga master named Sōchō, Tsunafusa and another retainer named Yokote 繁世 presented their works of poetry.  Thereafter, he received the daughter of Yokote Ippaku as a consort.  In 1512, during the Utsunomiya Disturbance, Tsunafusa followed his father and supported Shigetsuna.  In 1520, at the Battle of Johōji-Nawatsurushi, Tsunafusa contributed including with a stratagem to topple the base of the Nasu clan at Yamada Castle.  In 1523, at the Battle of Kawarada against the Minagawa clan, he opposed an invasion by the Utsunomiya of the territory of the Minagawa and severed the road for retreat by the Utsunomiya army.  That same year, his lord, Utsunomiya Tadatsuna, suffered a major defeat at the Battle of Saruyama.  Then, Haga Okitsuna (the third son of Shigetsuna later known as Utsunomiya Okitsuna), backed by the faction opposed to Tadatsuna including Haga Takatsune and Haga Takataka, forcibly took over the clan and Utsunomiya Castle.  This event is known as the Daiei Discord.  At this time, Tsunafusa was at his base protecting Tadatsuna.  Later, however, Tadatsuna suddenly.  This is also said to have been the result of an assassination after Tsunafusa abandoned him.

After the demise of Tadatsuna, Tsunafusa showed his ambitions, attempting to seize control of the area of Nikkōsan.  He sent his second son, Zazenji-Masayoshi and assigned him to the highest position in Nikkōsan known as orusushiki serving as Tsunafusa’s representative in the area.  In the Kyōroku era (1528 to 1532), he became the ruler of Nikkōsan and engaged in development activities including the Imamiya Shrine in Kanuma.

At this time, his lord, Utsunomiya Okitsuna, backed by the faction opposed to Tadatsuna, was a youth of ten years old utilized as a puppet by Haga Takatsune who regained the authority lost during the Utsunomiya Disturbance.  Tsunafusa took advantage of circumstances to expand his own base of power and, during the era of Okitsuna, solidified his position as a shukurō, or clan elder.  After Okitsuna matured and began to pursue an independent leadership, Tsunafusa joined Takatsune in opposition to Okitsuna.  Through the mediation of retainers of the Utsunomiya such as the Akahani and the Tomatsuri clans, he initially settled but tensions reemerged.  In 1532, Tsunafusa joined with Takatsune and Haga Takataka to pursue Okitsuna based on the crime of taking over the lord’s family, compelled his retirement, and backed Shigetsuna’s second son, Toshitsuna (Hisatsuna), as the successor.  In 1536, Tsunafusa, together with Takatsune, forced Okitsuna to take his own life.  In 1532, Tsunafusa conducted major renovations to Kanuma Castle and moved from a location in the mountains to the plains.

During the Tenbun era (1532 to 1555), thru the expansion of power driven by Tsunafusa, the Mibu clan attained a level of status next after the Haga and Mashiko clans.  Oyama Takatomo, the head of the Oyama clan neighboring the territory of the Mibu, even sent a sorrowful letter to Yūki Yoshitsuna of the Shirakawa-Yūki clan.  Takatsune, however, felt threatened by Tsunafusa and Tsunafusa approached his lord, Toshitsuna (who had a falling out with Takatsune over the incident by which Okitsuna took his own life) in opposition to Takatsune.  Tsunafusa then spread rumors of a revolt by Takatsune.  After Takatsune began to forge a relationship with the Oyama clan who were opposed to the Utsunomiya, in 1541, Tsunafusa joined with the Koga kubō (Ashikaga Haruuji), Oda Masaharu, and Satake Yoshiatsu to decimate Takatasune.  In 1542, his second son, Mibu Zazenji-Masayoshi, became dissatisfied with the overextension of the power of the Mibu in the area of Nikkōsan and retired to the village of Awano.  Tsunafusa responded by fabricating a story that Masayoshi was plotting a rebellion and sent his lineal heir, Tsunatake, to attack and eliminate him.  After the elimination of Takatsune, there was no one remaining who could suppress Tsunafusa so the Mibu clan wielded major influence in the Utsunomiya clan.

In 1549, after the death of his lord, Utsunomiya Hisatsuna, at the Battle of Kitsuregawa-Sōtemezaka, Tsunafusa exploited the chaos to occupy Utsunomiya Castle.  He settled with the Nasu clan and invited Takatsune’s son, Haga Takateru, to Utsunomiya Castle to establish a joint form of governance.  Nevertheless, Tsunafusa was the real holder of power, dissatisfying Takateru.  Tsunafusa assigned his lineal heir, Mibu Tsunatake, to Mibu Castle and his younger brother, Mibu Kanetake, to his base at Kanuma Castle.  He further promised to the lords of other castles in his territory to recognize their rights to their landholdings.  By this means, he strengthened the protection around him.  His only remaining antagonist was Haga Takasada who protected Ise Jumaru (the son of Hisatsuna later known as Utsunomiya Hirotsuna).  In 1551, when the Koga kubō demanded reinforcements from Tsunafusa to suppress a revolt by the Yūki clan, Tsunafusa dispatched his lineal heir, Tsunatake, as the commander-in-chief of 3,000 soldiers, contributing to a major victory.  These results demonstrated the power of the Mibu clan to those in surrounding provinces.  Tsunafusa steadily invaded the former territory of the Utsunomiya, expanding his domain and containing opponents from the Takō, Imaizumi, and Haga clans.  Meanwhile, the Shionoya clan assumed a neutral posture.

In 1555, Tsunafusa suddenly died.  He may have been murdered by Haga Takasada.