Lifespan: Eiroku 7 (1564 to 2/10 of Keichō 10 (1605)
Lord: Tamura Kiyoaki → Sōma Yoshitane → Uesugi Kagekatsu
Father: Ishikawa Arinobu
Siblings: Mitsumasa, Masahiro, Mitsuaki
Wife: [Formal] Adopted daughter of Kōriyama Fujimune
Ishikawa Mitsumasa served as a bushō during the Azuchi-Momoyama and early Edo periods. Mitsumasa was the lord of Dōmeki Castle in the Adachi District of Mutsu Province. His common name was Ishikawa Danjō.
In 1564, Mitsumasa was born as the son of Ishikawa Arinobu, the lord of Dōmeki Castle. The Dōmeki-Ishikawa clan served as senior retainers of the Shiomatsu-Ishibashi clan, and were a branch of the Mutsu-Ishikawa clan who served as the lords of Miyoshi Castle. The clan was founded by Morimitsu, the third son of Mitsutomo, the seventeenth head of the Mutsu-Ishikawa clan. Mitsumasa’s aunt was the formal wife of the twenty-fourth head of the family – a daimyō named Ishikawa Harumitsu.
In 1568, Mitsumasa’s father, Arinobu, along with Ōuchi Yoshitsuna and others, colluded with the Tamura clan, ousted his lord, Ishibashi Hisayoshi. Thereafter, he affiliated with the Tamura clan. Later, Yoshitsuna’s son, Ōuchi Sadatsuna, aimed to become independent of the Tamura clan but Mitsumasa did not go along with the plan. In 1585, Mitsumasa followed the lead of Tamura Kiyoaki and his son-in-law, Date Masamune, to attack Sadatsuna. Owing to his contributions, Mitsumasa was granted Odemori Castle by Masamune and moved there, but, after the death of Kiyoaki, he was dissatisfied to be assigned as a yoriki, or security staff, to Shiroishi Munezane, the lord of Miyamori Castle. In 1588, at the Battle of Kōriyama, Mitsumasa colluded with Sōma Yoshitane, triggering an attack by Masamune on his base at Odemori Castle. In addition to losing the castle, over 500 soldiers were killed while Mitsumasa himself fled to the protection of Yoshitane.
In 1600, at the Battle of Sekigahara, Mitsumasa served Uesugi Kagekatsu who controlled the Adachi District and participated in an attack against Mogami Yoshiaki but, after the defeat of the Uesugi army, he incurred a reduction in his fief, separated from the Uesugi family, and became a rōnin, or masterless samurai.
He died on 2/10 of Keichō 10 (1605) at the age of forty-two.