The son of Nejime Kiyotoshi, Shigenaga was a retainer of the Kimotsuki clan, and the 16th generation head of his family.
However, following the Kimotsuki clan's defeat, Shigenaga saw that the family's position was untenable, so he independently concluded a peace agreement with Shimazu Yoshihisa.
[2] This incurred the wrath of Kimotsuki Kanesuke, who attacked him; however, the Shimazu were able to save Shigenaga from death.
Shigenaga is said to be the first person who encouraged the cultivation of mandarin oranges in Japan.
Shigenaga was the ancestor of Komatsu Tatewaki, the famous Bakumatsu-era senior councilor of Satsuma han.