She made her professional debut in 1955 with Akakke Kōma Pony, a Western story with a tomboy heroine.
[2][3] In 1952 at the age of 12, she regularly contributed to competitions organized by the monthly magazine Manga Shōnen chaired by Tezuka.
Although her manga was never accepted, her efforts did not go unnoticed: she received an honorable mention and publisher Akira Maruyama from Kōdansha took notice.
[4][3] For a year and a half, Mizuno worked to make a living and drew for Shōjo Club at the same time.
[6] In 1958, invited by Tezuka, she moved to Tokyo in the Tokiwa-sō apartment where she lived and worked with the two authors Shōtarō Ishinomori and Fujio Akatsuka, together they collaborated on two manga under the pseudonym U. Mia for the magazine Shōjo Club.