Mizumachi was born in Imari, Saga Prefecture,[4][5][6] Japan and started playing basketball while in elementary school.

[7] After graduating from Gakushuin University in 2004 he decided to enter professional basketball, against his parents' wishes.

In June 2010 Mizumachi was signed by the Happinets in the second round of the 2010 expansion draft and has served as captain throughout the club's seven seasons in the bj league.

[7] He has remained with Akita following the team's entry into the B.League in September 2016, but has been replaced as captain by Shigehiro Taguchi.

[10] His retirement ceremony was held at the Akita Prefectural Gymnasium on April 3, 2019.