Kakiya Mitsunari served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was a retainer of the Yamana clan.
Mitsunari was born as the son of Kakiya Tsugunari or, under another theory, as the son of Kakiya Tsugutsura (in which case Tsugutsura was the son of Tsugunari). He is deemed to have died in 1582 so accounts of his exploits thereafter are said to be in regard to his son, Kakiya Tsunefusa.
As a powerful senior retainer of Yamana Suketoyo, Mitsunari wielded authority in the era of Tsugunari. In 1570, after Tsugunari was killed by Tainoshō Koreyoshi in the course of ongoing conflict, in 1575, Mitsunari subsequently killed Koreyoshi.
In 1575, after his lord, Suketoyo, betrayed the Oda and reconciled with the Mōri clan, Mitsunari cut ties with Suketoyo.
In 1580, when, acting upon orders of Nobunaga, Hashiba Hideyoshi invaded Tajima, Mitsunari quickly surrendered. Following the pacification of Tajima by the Hashiba forces, Mitsunari participated in an assault against Yamana Toyokuni who was holed-up at Tottori Castle. Owing to his contributions, Mitsunari received a fief of 20,000 koku in the Iwai District of Inaba Province and became the lord of Futagamiyama Castle.