Lifespan: Genki 3 (1572) to 10/15 of Keichō 13 (1608)
Other Names: Shirōbei (common)
Lord: Mori Tadamasa
Father: Kagami Motomasa
Siblings: Motomine, Masayasu, Kii (wife of Mori Shigemasa), sister (consort of Kyōgoku Takatomo)
Cousins: Masayasu (or Tadayasu)
Wife: [Formal] Daughter of Hayashi Tametada
Kagami Motomine served as a bushō from the Sengoku to early Edo periods. He was the chief retainer of the Tsuyama domain in Mimasaka Province and the chamberlain of Takada Castle (Katsuyama Castle).
In 1572, Motomine was born as the eldest son of Kagami Motomasa, the head of the Mino-Kagami clan and a retainer of the Mori family.
Motomine served Mori Tadamasa. As the head of retainers of the Mori family, Motomine received a stipend of 8,000 koku. In 1608, Motomine got into a fight with another individual named Ozawa Hikohachi at a quarry on Mount Ishi in the village of Ōtani and killed him. Hosono Sahyōe mediated only to then be murdered by one of Motomine’s retainers. The next day, Motomine took responsibility and committed seppuku. After hearing the story, Tadamasa determined both parties to the fight were guilty and he seized the stipends of three families. Later, Motomine’s grandson, Kagami Masanao, was engaged in service for 2,000 koku. Motomine’s great-great-grandson, Kagami Seiemon was adopted by the family of his lord and became Mori Naganari, the third lord of the Akō domain based in the Akō District of Harima Province.