Lifespan: Tenbun 13 (1544) to 11/19 of Genna 1 (1616)
Rank: bushō, daimyō
Title: Junior Fifth Rank (Lower), Governor of Bungo
Lord: Saitō Dōsan → Saitō Yoshitatsu → Saitō Tatsuoki → Oda Nobunaga → Toyotomi Hideyoshi → Toyotomi Hideyori → Tokugawa Ieyasu → Tokugawa Hidetada
Father: Nishio Nobumitsu or Nishio Mitsumasa
Siblings: Mitsunori, Sadanori, sister (wife of Takagi Sadatoshi)
Wife: [Formal] Daughter of Ujiie Yukitaka (younger sister of Ujiie Naomoto)
Children: Daughter (second wife of Hori Naokage), daughter (wife of member of the Takagi clan, later, wife of Murase Shigeharu), daughter (wife of Kinoshita Yoshitaka)
Adopted Children: Yoritsugu, Yoshinori, Ujinori
Nishio Mitsunori served as a bushō and daimyō from the Sengoku to early Edo periods.
In the Edo period, Mitsunori served as the first lord of the Mino-Ibi domain. The Nishio clan originated from Mikawa Province, or he may have been a member of the Momii clan of Tanba Province. His relationship to Nishio Naonori who served the Saitō clan of Mino in the Sengoku period is uncertain.
Initially, Mitsunori was a servant of the Ujiie clan. He then served Saitō Dōsan of Mino, and then Oda Nobunaga of Owari. After a coup d’état against Oda Nobunaga on 6/2 of Tenshō 10 (1582), known as the Honnō Temple Incident, Mitsunori was solicited by Akechi Mitsuhide to betray the Oda, but refused. He then served Hashiba Hideyoshi. Around this time, he governed a fief of 20,000 koku from Sone Castle in Mino. After the demise of Hideyoshi in the eighth month of 1598, Mitsunori approached Tokugawa Ieyasu. In 1600, prior to the Battle of Sekigahara, Mitsunori was solicited by Ōtani Yoshitsugu to join the Western Army, but he refused and joined the Eastern Army instead, after which Yoshitsugu burned Mitsunori’s land. Prior to the main Battle of Sekigahara, Mitsunori participated in the Siege of Gifu Castle and the Siege of Ōgaki Castle, and, owing to his detailed knowledge of the local terrain, served as a guide in Mino. After the war, he was awarded an increase to his fief of 10,000 koku and became the founder of the Ibi domain of 30,000 koku in Mino.
Later, he allocated 5,000 koku to his grandchild from a daughter married into another family, Nishio Yoshinori. From 1614, Mitsunori served valorously in the Siege of Ōsaka, but after the Summer Campaign, on 11/19 of Genna 1 (1616), he died in Sunpu.
Mitsunori did not have an heir, so he was succeeded by Yoshinori.