Nicknamed "Mighty Mouse" partly because of her small stature (standing barely five feet tall) and partly due to her competitive drive, Tanner had a large impact on Canadian swimming and is considered one of the top performers in the sport.
During the 1966 Commonwealth Games in Kingston, Jamaica, Tanner won four gold medals and three silvers, becoming the first woman to ever win four golds at a Commonwealth Games. She was named Canada’s best athlete in 1966 — the youngest person to ever receive the award — and was also selected this country’s top athlete overall. The following year at the 1967 Pan American Games, Tanner won two gold and three silver medals in Winnipeg, breaking two world records in the process. Tanner arrived at the 1968 Summer Olympics as a heavy medal favorite. She earned three times in Mexico City, winning two individual silver medals and one relay bronze. Suffering from depression, Tanner retired from competition after the 1968 Olympics at just 18 years of age.