At 15 years, 311 days, while attending a high school in Hamamatsu Yasuji Miyazaki became the youngest swimmer to win the 100 metre freestyle, also ending the US winning streak that started in 1906. In the semi-final he managed to break Johnny Weissmüller’s Olympic Record with 58.0. He also won a gold medal in the 4x200 freestyle relay, swimming the first leg as a substitute for Tsutomi Oyokota as the team set a world record. One year before the Olympics, he was the first swimmer to win the national title in the 100 m freestyle in a time of less than one minute (59.2). Following silver medalist Tatsugo Kawaishi, Miyazaki enrolled in the Faculty of Law at Keio University, where he served as captain of the swimming team. Although he was initially selected for the 1936 team, he never took part. At home, he won the Japanese Championship in the 100 m freestyle in 1931.
In 1981 Miyazaki was inducted into the International Swimming Hall of Fame, and in 1989 he was awarded the Olympic Order in Silver. After his Olympic participation, he studied law in Tokyo and later lived in Nishinomiya, Hyogo. After the war, he was director of the Japanese Swimming Federation.