Kawachi no Kami Nagakuni is said to have been born in Echizen Province in Kan'ei 10 (1633), a date deduced from an extant blade signed "Kanbun 8, second month, day -- made at age thirty-six." Traditions variously state that he studied in Edo under Hojoji Kunimasa or under Higo no Kami Yoshitsugu, and that he later forged at Kumamoto in Higo Province, likely at the invitation of the Hosokawa family. The details of his training are not firmly established, yet extant works inscribed "O Hishu Kumamoto" constitute direct evidence of his movements.

Nagakuni's forging characteristically shows tightly packed ko-itame-hada mixed with mokume; fine-granular ji-nie adheres thickly, and fine chikei enters well. His hamon typically presents gunome linking together into midare, mixed with ko-notare, ko-gunome, and pointed elements; the nioi is deep, ko-nie adheres thickly, and patches of yubashiri and tobiyaki intermingle. Sunagashi runs through, kinsuji and nie-suji enter well, and the nioiguchi is bright. The manner in which the tang's file marks begin shallowly and gradually deepen into o-sujikai, together with the overall nakago finishing, shows affinities with Yamato no Kami Yasusada of the same province. The NBTHK setsumei observe that the linked gunome construction and thickly adhering ko-nie may suggest that Nagakuni belonged to the Yasusada group rather than the Hojoji lineage, though this must await further research.

Surviving examples by Nagakuni are relatively few, and his best works are praised for an especially refined forging and a brightness of nioiguchi that calls to mind the styles of contemporaneous Edo-period masters such as Kotetsu and Kazusa no Suke Kaneshige. A wakizashi with kiriba-zukuri on the omote and shinogi-zukuri on the ura, inscribed as made with nanban-tetsu, demonstrates the breadth of his expressive range.