Reizei Mototoyo served as a bushō during the Sengoku and Azuchi-Momoyama periods.  He was a retainer of the Ōuchi and Mōri clans.

The Reizei were originally the Tatara clan, a branch of the Ōuchi clan.  Takatoyo’s father, Reizei Okitoyo, adopted the surname of his wife (from the Reizei family) and thereafter they were known as the Reizei clan.

In 1538, Mototoyo was born as the son of Reizei Takatoyo, a retainer of the Ōuchi clan.  During his coming-of-age ceremony, he was conferred one of the characters from the name of Mōri Motonari and adopted the name of Mototoyo.

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.

Mototoyo and his younger brother, Reizei Motomitsu, 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, Mototoyo and Motomitsu served the Mōri clan.

After the Subjugation of Bōchō, the Mōri army advanced into northern Kyūshū and occupied the northern portion of Buzen Province.  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.  He was twenty-five years old.  Mototoyo did not have a son so his younger brother, Motomitsu, inherited the headship of the Reizei clan.  After the Mōri recaptured Moji Castle, Motomitsu because the chamberlain.