Iwanari Tomomichi served as a bushō during the Sengoku and Azuchi-Momoyama periods. Tomomichi was a retainer of the Miyoshi clan and member of the Miyoshi Group of Three (along with Miyoshi Nagayasu and Miyoshi Sōi). From 1570, in the last several years of his life, he used the name of Naganobu.
Tomomichi gained prominence as a magistrate of Miyoshi Nagayoshi, becoming a central figure in the Miyoshi administration. He is regarded as the most successful individual in that administration.
Tomomichi’s origins are uncertain. He may have been from Yamato Province given that, through much of the Muromachi period, the Iwanari Shrine served as an auxiliary shrine to the Isonokami Shrine in Yamato which was governed by the Hosokawa clan. He may also have been related to the dogō, or small-scale landowners, based on the place name of the Iwanari neighborhood in the Honji District of Bingo Province. Based on one account, he worked as an administrator in Nishikujō in the environs of Kyōto and, thereafter, became a retainer of the Miyoshi clan. The surname of Iwanari appears in a letter sent by a retainer of Miyoshi Motonaga named Shiota Tanemitsu noting that an administrator under the name of Iwanari had seized the manor in Nishikujō, therefore, there is a belief that, similar to Matsunaga Hisahide, Tomomichi came into service in the Kinai region.
In historical accounts, Tomomichi’s name first appears in 1550 where he conducted an inquiry into a dispute concerning carpentry work at the Kitano Shrine. In the eleventh month of 1551, he attended a tea ceremony at the Tennōji house in Sakai of Tsuda Sōtatsu, the father of Tsuda Sōgyū – a merchant and tea master in Sakai. Thereafter, Tomomichi served as a magistrate under Miyoshi Nagayoshi. In 1558, he participated in the occupation of Shōguyama Castle at the Battle of Kitashirakawa. This is the first recorded military action for Tomomichi. In 1562, when Rokkaku Yoshikata invaded Kyōto, Tomomichi served as a guard for Ashikaga Yoshiteru (the thirteenth shōgun of the Muromachi bakufu).
After the death of Nagayoshi, he served as the guardian of Nagayoshi’s nephew, Miyoshi Yoshitsugu. In 1565, Tomomichi joined the other two members of the Miyoshi Group of Three to assassinate Ashikaga Yoshiteru in an event known as the Eiroku Incident. He fought frequently against Matsunaga Hisahide and Hatakeyama Takamasa.
In 1566, Tomomichi attacked dogō including Nakazawa Michifusa and Kawashima Kazunori holed-up in Shōryūji Castle in Yamashiro. After toppling the castle, Tomomichi fiercely pursued other dogō opposed to him, expelling the Kawashima and other families. He gave most of the captured territory to new landowners. Meanwhile, he entered Shōryūji Castle and governed Nishioka in the western portion of Yamashiro. Based in Shōryūji Castle, he endeavored to solidify a new governance in the Nishioka area and was praised for adopting innovative practices. As for Shōryūji Castle, it was sufficient for dogō to take refuge in time of conflict, but, after entering, he organized the premises to serve as a base for his administration that brought together the dogō in the area. From this perspective, Tomomichi is regarded as the first lord of Shōryūji Castle.
In 1567, Tomomichi joined Ikeda Katsumasa to lead an army of 10,000 soldiers to confront Matsunaga Hisahide at the Tōdai Temple in Yamato. A surprise attack by Hisahide resulted in defeat at the Battle at the Giant Buddha of Tōdai Temple. In the ninth month of 1568, when Nobunaga marched upon Kyōto to install Ashikaga Yoshiaki as the fifteenth shōgun, Tomomichi combined forces with Miyoshi Nagayasu, Miyoshi Sōi, and Shinohara Nagafusa, as well as a former enemy, Rokkaku Yoshikata, to mount a fierce resistance. However, after an attack upon his base in an event known as the Siege of Shōryūji Castle, Tomomichi was compelled to retreat. While other castles across the Kinai were surrendered without much resistance, only Tomomichi in Shōryūji Castle and Ikeda Katsumasa in Ikeda Castle in Settsu put-up a stiff resistance. This served as evidence of the fruition of his governance whereby the dogō in his territory came together under his command. On 9/27 of Eiroku 11 (1568), Tomomichi resisted Nobunaga from Shōryūji, but the castle fell on 9/29. On 1/8 of Eiroku 12 (1569), Hosokawa Fujitaka entered Shōryūji. Until then, Tomomichi was the lord of the castle and he received a gift from the Kōgen Temple. Meanwhile, in the first month of 1569, he attempted an attack against Ashikaga Yoshiaki and his bakufu forces, but was repelled in an event known as the Battle of Honkoku Temple.
Thereafter, he served Nobunaga. In a letter to Hosokawa Fujitaka, Nobunaga expressed his trust in Tomomichi, referring to him as straightforward in character. However, after Ashikaga Yoshiaki sent orders to numerous daimyō to oppose Nobunaga for the Encirclement of Nobunaga, Tomomichi reacted by once again confronting Nobunaga. In the eighth month of 1573, Tomomichi was subject to a siege at Yodoko Castle by the Oda army including Hosokawa Fujitaka and Mitsubuchi Fujihide (Fujitaka’s younger brother of a different mother). Betrayed by Bangashira Ohohi-no-kami Yoshimoto and Suwa Hida-no-kami, Tomomichi charged out of the castle to fight the besieging army, where he clashed with a retainer of Fujitaka named Oritsu Gonnai. After falling down, he was killed in the water. This was known as the Second Siege of Yodoko Castle. Despite having defensive measures in place, the decision to engage the besieging army may have been encouraged by Bangashira Ohohi-no-kami Yoshimoto and Suwa Hida-no-kami owing to their collusion with Hideyoshi.
Although not certain, he is surmised to have been approximately forty-three years old. Along with Tomomichi, 340 soldiers in his army were killed in action. His head was then brought to Nobunaga while on a march to Takashima in Ōmi Province. According to the Shinchō-kōki, Nobunaga praised Tomomochi for his unparalleled valor and covered the head with his undergarment.
Among the other members of the Miyoshi Group of Three, Miyoshi Nagayasu went missing, while Miyoshi Sōi had already died. Consequently, the group came to an end.
In the chronicles of Luís Fróis, a Portuguese missionary in Japan during this period, Tomomichi and Miyoshi Sōi were called enemies of the laws of God. Unlike Miyoshi Nagayasu who showed an understanding of Christianity and was called a friend of the church, Tomomichi was not tolerant of the Christian religion.