Mizawa Tamekiyo served as a bushō during the Sengoku and Azuchi-Momoyama periods.  Initially, Tamekiyo served the Amago clan, and, later, the Mōri clan.

The Mizawa were descendants of the Iijima clan, kokujin, or provincial landowners, in southern Shinano or descendants of Minamoto no Yoshinaka, a bushō from the latter part of the Heian period.  Tamekiyo served as the head of the Mizawa clan, a kokujin based at Mizawa Castle in the Nita District of Izumo Province.

In 1536, Tamekiyo was born as the son of Mizawa Tameyuki, the head of the Mizawa clan, a kokujin, or provincial landowner, in Izumo.  From 1540 to 1541, Tameyuki joined the Amago in the Siege of Yoshida-Kōriyama Castle and was killed in action so Tamekiyo succeeded him as head of the clan while still in his youth.

After the Amago clan lost at the Siege of Yoshida-Kōriyama Castle, the Mizawa clan established friendly relations with Ōuchi Yoshitaka.  During the First Siege of Gassantoda Castle from 1542 to 1543, Tamekiyo sided with Yoshitaka.  However, the assault on Gassantoda Castle ran into difficulties, so Tamekiyo, along with Kikkawa Okitsune, Mitoya Hisasuke, Honjō Tsunemitsu, and Yamauchi Takamichi returned to support the Amago again.

In 1562, after Mōri Motonari launched the invasion of Izumo, Tamekiyo returned to the service of the Mōri and received recognition of his rights to his original territory in addition to lands in the Yokota manor in the Nita District of Izumo that was taken from the Amago clan.  Thereafter, he moved his base to Fujigase Castle in the Yokota manor and served on behalf of the Mōri.

In 1569, during the invasion of Kyūshū by the Mōri, Tamekiyo deployed under the command of Kikkawa Motoharu, participating in the Battle of Tachibana against the Ōtomo clan in Chikuzen Province.  The Mōri attacked Tachibanayama Castle, but were defeated in the fifth month at the Battle of Tatarahama.  Following the loss, and owing to the Revolt of Ōuchi Teruhiro and the revival of the Amago clan in Izumo, the Mōri forces were compelled to retreat from Kyūshū.

While the main division of the Mōri army was in Kyūshū, Amago Katsuhisa and Yamanaka Yukimori took advantage of their absence to invade Izumo and attacked Gassantoda Castle defended by Amano Takashige.  When Kobayakawa Takakage learned of these developments from Tachibanayama Castle, he ordered Tamekiyo and his son, Mizawa Tametora, along with Yonebara Tsunahiro, Amano Takehiro (Takashige’s son), and Saka Shōyūrokurō to return as reinforcements for the troops at Gassantoda.  At this time, Tamekiyo was solicited by Tsunahiro to rebel against the Mōri based on a secret letter that Tsunahiro received from Ōtomo Sōrin urging him to ally with Amago Katsuhisa.  Tamekiyo, however, refused the offer and entered Gassantoda Castle.  After returning to Izumo, Tamekiyo and Tametora intercepted the Amago revival army and, in 1570, served valorously at the Battle of Fubeyama, defeating revival army.  After achieving this victory, Tamekiyo continued to fight under the command of Kikkawa Motoharu in Mimasaka and in the Sanin Region.  Owing to these contributions, he received from Motonari and Terumoto a fief totaling 2,4000 kan in Izumo, Oki, and Hōki provinces.

In 1569, Tamekiyo exchanged written oaths with Motonari and Terumoto, in 1571, he made a written contract with Kikkawa Motoharu to act as siblings.  After returning to the service of the Mōri, he demonstrated a higher degree of autonomy.  His eldest son, Mizawa Tametora, also submitted written oaths to the Mōri on several occasions, including in the first and sixth months of 1580 and in 1585.

In 1573, he established his base at Kamedake Castle in Kamedake in the Nita District of Izumo and, in 1574, transferred the headship of the clan to his eldest son, Tametora.

Tamekiyo died in 1588 at the age of fifty-three.  His daughter wed Mōri Motoaki, the fifth son of Mōri Motonari.