Duffy is the 2016 and 2017 ITU World Triathlon Series World Champion, the 2015 and 2016 ITU Cross Triathlon World Champion, and a five-time winner (2014-2017, 2019) of the XTERRA World Championships. She competed for Bermuda at the 2008 Summer Olympics in Beijing, the 2012 Summer Olympics in London, the 2016 Summer Olympics in Rio de Janeiro, and the 2020 Summer Olympics in Tokyo, where she won gold. It was Duffy's first Olympic medal, as well as Bermuda's first gold medal. Duffy's medal also made Bermuda the smallest country in the world to have won a gold medal at the Summer Olympics.