As part of the 2021 Canadian Olympic swimming trials in Toronto, Mastromatteo won the 100 m breaststroke race. This qualified him for the 2020 Summer Olympics in Tokyo. He placed thirty-eighth in the heats of the 100 m breaststroke, his lone individual event. He stated his main focus was on the 4x100 m medley relay, where the Canadian men's team made the final and placed seventh, with Mastromatteo swimming a new personal best relay time on the breaststroke leg.