The Ōki clan originated from Ōki Masanaga, the son of Utsunomiya Tomotsuna who was the younger brother of Kamachi Hisanori of the Chikugo-Utsunomiya clan in the early Muromachi period.  Munemitsu was a descendant of Masanaga.  In the era of Munemitsu, Kamachi Shigenami served as the seventeenth head of the Kamachi clan.  Shigenami’s mother, 乙鶴姫, was the older sister of Tajiri Akitane while Munemitsu’s mother was the younger sister of Akitane.  Consequently, Munemitsu was a cousin of Shigenami and served as a chief retainer.

Shigenami was under the command of Ryūzōji Takaknobu of Hizen Province.  Shigenami and Takanobu had a falling out, during which period Takanobu feared that Shigenami would abandon the Ryūzōji in favor of the Shimazu clan.  Takanobu then conspired to murder Shigenami, sending a messenger to Shigenami’s base in Yanagawa to persuade Shigenami to come to Hizen under the pretext of attending a banquet.  Although Shigenami initially refused, the messenger eventually persuaded him to attend despite warnings from Munemitsu not to travel to Hizen.  Beginning with his older brother, Kamachi Shigehisa, Shigenami chose 200 capable retainers to accompany him and then departed Yanagawa.

After traversing the Chikugo River and entering Hizen, Shigenami was welcomed by Takanobu’s eldest son, Ryūzōji Masaie, at Saga Castle.  The next day, a Ryūzōji battalion launched an unexpected attack against him near the Yoka Shrine.  The 200 elite forces accompanying Shigenami fought valiantly, but, outnumbered, Shigenami took his own life while Shigehisa and the rest of the party were all killed in action.  After learning that Shigenami had been killed, Munemitsu went to the base of Nabeshima Naoshige in Saga and attempted to commit seppuku to follow Shigenami into the afterlife, but was stopped from doing so and became a rōnin, or wandering samurai.

Later, Munemitsu was engaged by the Nabeshima family and, in the Edo period, his descendants became senior figures of the Saga domain.  Thereafter, another descendant, Ōki Takatō, was a politician of noble rank during the Meiji period.