Matsumoto Domain
Nagakuni was the eldest son of Matsudaira Mitsutsune, the daimyo of Matsumoto Domain in Shinano province. In October 1840, when the 4th daimyo of Karatsu, Ogasawara Nagayoshi died without heirs, the Tokugawa Bakufu selected him as his replacement, and he was posthumously adopted as Nagayoshi’s son.