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 and the first person to get seven medals in those games. She won the Lou Marsh Trophy, recognizing her as Canada's best athlete in 1966 — the youngest person to ever receive the award — and was also selected as the country's top athlete overall. The following year at the 1967 Pan American Games in Winnipeg, Tanner won two gold and three silver medals, breaking two world records in the process. Tanner arrived at the 1968 Summer Olympics in Mexico City as a heavy medal favorite. She won three Olympic medals in Mexico City, including two individual silver medals and one relay bronze. However, the media deemed the lack of gold a disappointment and led Tanner to suffer from depression, retiring from competition after the 1968 Olympics at just 18 years of age.