Reizei Motomitsu served as a bushō during the Sengoku and Azuchi-Momoyama periods.  He was a retainer of the Mōri clan.

In 1541, Motomitsu was born as the son of Reizei Takatoyo, a retainer of the Ōuchi clan.

On 9/1 of Tenbun 20 (1551), during the Tainei Temple Incident, Motomitsu’s father, Takatoyo, accompanied his lord, Ōuchi Yoshitaka, to the end and died when cornered by the Sue army at the temple.

A young Motomitsu and his older brother, Gorō (later known as Reizei Mototoyo) were together taken by their uncle, Yoshiyasu Toyohide, and fled to the territory of Hiraga Hiroyasu, the former head of the Hiraga clan, a kokujin, or provincial landowner, in Aki Province.  Thereafter, during the Subjugation of Bōchō by Mōri Motonari from 1555 to 1557, Motomitsu and Mototoyo served the Mōri clan.  The Reizei clan served as navy commanders for generations so Motomitsu also served as a commander of the Mōri navy and engaged in battle against the Ōtomo and Oda clans. Suō

Mototoyo was appointed to serve as the chamberlain of Moji Castle in Buzen, overlooking the Kanmon Straits.  On 10/13 of Eiroku 5 (1562), Tachibana Dōsetsu led Ōtomo forces in an attack during which Mototoyo, along with Katsura Motochika and Arakawa Motoyoshi were killed at Yagiura and Dairi in Buzen.  This is known as the Siege of Moji Castle. 

Mototoyo did not have a son so Motomitsu, as his younger brother, inherited the role as the chamberlain of Moji Castle in addition to the headship of the Reizei clan.  Motomitsu then wed as his formal wife the daughter of Shibukawa Yoshimitsu, a member of the Shibukawa family serving as the governor-general of Kyūshū.  According to one theory, this wedding occurred so that Motomitsu would become an adopted son-in-law and inherit the headship of the Shibukawa clan.  The character of “mitsu” in the name of Motomitsu is surmised to have originated from his adoptive father, Shibukawa Yoshimitsu.  Nevertheless, he inherited the Reizei clan.  Meanwhile, the character of “moto” in his name is regarded as having originated from Mōri Terumoto.  As such, when he adopted the name of Motomitsu, this would have been after Terumoto underwent his coming-of-age ceremony in 1565.

In 1589, Motomitsu was a mistress of Mōri Terumoto named Yokota-no-tsubone.  At this time, Terumoto granted to Yokota-no-tsubone (rather than Motomitsu) landholdings of 100 koku in the Asa District of Nagato Province.

During the Bunroku Campaign from 1592, Motomitsu followed Terumoto to the Korean Peninsula.  After returning to Japan, in 1594, Motomitsu became the lord of Kamedake Castle in the Nita District of Izumo Province.

In 1597, for the Keichō Campaign, Motomitsu again followed Terumoto to the Korean Peninsula.  On 12/22 of Keichō 2 (1597), at the First Siege of Ulsan Castle, he incurred an attack by 1,000 soldiers led by 擺寨 in the vanguard of the Chinese army.  Motomitsu, along with 137 soldiers under his command including Asonuma Motohide and Tsuno Ieyori (retainers of the Mōri) died in battle.  He was fifty-eight years old.  The remains of Motomitsu and other retainers of the Reizei clan were placed in a ship by Motomitsu’s cousin, Yoshiyasu Mitsusada, and repatriated to Japan.  Later, Mitsusada returned to Ulsan Castle and on 12/23, was killed defending the site.

His lineal heir, Reizei Motonaga, inherited the headship of the clan.