Nagasawa Mitsukuni served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was a retainer of the Uesugi clan.
In the Kamakura period, the Nagasawa clan settled in Etchū Province as kokujin, or provincial landowners, based in Nagasawa in the Nei District. The Nagasawa were called a branch of the Toki clan descended from the Seiwa-Genji. In the Nanboku period, the clan served under a shugo daimyō named Momonoi Tadatsune. Thereafter, in the Muromachi period, the clan served as members of the hōkōshū, or military organ under the direct control of the shōgun. In the Sengoku period, the Nagasawa clan extended their power into the Himi area of Etchū, and Nagasawa Yoshinori, who served as the lord of Ebise Castle in Himi, is surmised to have been of the same family as Mitsukuni.
By the Tenshō era (1573 to 1593), as Uesugi Kenshin extended his authority into the western portion of Etchū Province, Mitsukuni served him by defending Yuyama Castle (Moridera Castle), a stronghold on the provincial border with Noto Province. In 1576, Mitsukuni participated in the pacification of Noto by the Uesugi army. After the fall of Nanao Castle, Mitsukuni contributed to the toppling of Matsunami Castle where Matsunami Yoshichika (from an illegitimate branch of the Noto-Hatakeyama) continued to resist. Mitsukuni then was assigned to be the commander in charge of Anamizu Castle in the Fugeshi District of Noto.
In 1578, after the sudden death of Uesugi Kenshin, Chō Tsuratatsu (acting upon orders of Oda Nobunaga) invaded Noto Province. Former retainers of the Hatakeyama such as Nukui Kagetaka and Miyake Nagamori abandoned the Uesugi. While fighting against them, Mitsukuni and his son, Shichijirō, were killed at Mount Sekidō.
A group of stone Buddha statues donated by Mitsukuni in 1572 remain at the Jōnichi Temple of the Shingon sect in the city of Himi in Ishikawa Prefecture.