Niiro Tadamitsu served as a bushi during the Azuchi-Momoyama and early Edo periods.  He was a retainer of the Shimazu clan.

Tadamitsu was born as the lineal heir of Niiro Tadataka.

Prior to the conduct of Tadamitsu’s coming-of-age ceremony, his father, Tadataka, died in 1583 during an assault against Fukae Castle (which was aligned with the Ryūzōji clan) in Hizen Province.

On 8/22 of Tenshō 14 (1586), Tadamitsu was crowned by Shimazu Yoshihisa at his coming-of-age ceremony held in Yatsushiro in Higo Province.  He received items to be worn from the waist and the same common name as his father, Jirō-shirō.

In 1587, as an outcome of the campaign by Toyotomi Hideyoshi known as the Subjugation of Kyūshū, the Shimazu clan surrendered.  In 1590, Tadamitsu traveled to Kyōto as a hostage sent by his grandfather, Niiro Tadamoto, and resided in Fushimi.  Later, Tadamoto’s younger brother, Niiro Tadasuke, took his place so Tadamitsu returned to Satsuma.

In 1603, Tadamitsu died.  He was succeeded by Niiro Tadakiyo, the lineal heir of Tadamitsu’s uncle, Niiro Tadamasu.  By this means, Tadakiyo became Tadamitsu’s son-in-law.