Mizushima was born into a wealthy family; her mother's family had a bookstore where she would consult novels. In 1960 (at the age of 13), she was introduced to Nobuko Yoshiya, a commercially successful novelist who specialized in serialized romance novels for teenage girls and is known to be the pioneer of Japanese lesbian literature. These serials are what made Mizushima want to become a novelist. At the age of 14, she contributes to the local newspaper and a magazine. At the age of 18, she enters the Japanese Women College in home economics. On September 1, 1923, the Great Kanto earthquake strikes and Mizushima is affected in her region.