In 2021, Stubblety-Cook competed in the men's 100m and 200m breaststroke events at the 2020 Summer Olympics in Tokyo, winning a gold medal (and setting an Olympic record) in the latter event.[6] He swam the breaststroke leg in the mixed 4x100m relay in which Australia finished 3rd, winning a bronze medal.[7] He trains under Vince Raleigh at the Chandler Aquatic Centre, Brisbane, Queensland.
He won his first international medal, a silver medal, in the 200m breaststroke, at the 2018 Pan Pacific Championships in Tokyo. He continued to impress at the 2021 Australian Swimming National Championships, taking out two national titles in the 100m breaststroke and 200m breaststroke.[9]
In 2023, he competed at the World Championships in Fukuoka and finished with a silver medal in the 200 metres breastroke after being 0.92 seconds behind Qin Haiyang, who had broken the world record set by Stubblety-Cook, during that event. He also took part in the 4×100 m mixed medley and 4×100 m medley events, and took silver and bronze respectively.[13][14][15]
Legend: OC – Oceanian record;NR – Australian record; Records not set in finals: h – heat;sf – semifinal;r – relay 1st leg;rh – relay heat 1st leg;b – B final;† – en route to final mark;tt – time trial