Mizuno Tadamoto served as a daimyō during the early Edo period.  Tadamoto was the first head of his branch of the Mizuno family and the first lord of the Shimōsa-Yamakawa domain.

Tadamoto was born as the third son of Mizuno Tadamori.  His formal wife was the daughter of Miura Tameharu, a retainer of the Kishū-Tokugawa family.

Tadamoto served as a close associate of Tokugawa Hidetada, the second shōgun of the Edo bakufu.  In 1605, he was invested with titles of Junior Fifth Rank (Lower) and Director of the Bureau of Keys and Facilities.  He also assumed the position of commander of the koshōkumi, an elite cavalry unit.  At this time, in addition to Tadamoto, there were five commanders of cavalry units including Kusakabe Masafuyu, Inoue Masanari, Itakura Shigemune, Naruse Masatake, and Ōkubo Noritaka.  

Tadamoto served in the Winter Campaign of the Siege of Ōsaka in 1614 and the Summer Campaign of the Siege of Ōsaka in 1615.  Later that year, he commanded a cavalry unit known as the shoinban based in the west citadel.  He was granted landholdings of 35,000 koku in Shimōsa, Shimotsuke, and Ōmi provinces, with the status of a daimyō as lord of the Shimōsa-Yamakawa domain.

Tadamoto died in 1620.  His grave is at the Kenkon Temple in the town of Higashiura in Aichi Prefecture.

As recognition for his service as the first head of his branch and daimyō, the Mizuno family commemorated him by obtaining permission to build graves in Yamakawa until the eleventh head of the family, Mizuno Tadakuni, even though this was not in his original territory.