2021–2022

In July 2021, Cooper competed at the Olympics in Tokyo. He finished twelfth in the 100 m backstroke in a personal best time of 53.43.^[4][5] He then competed in the heats of the mixed 4 × 100 m medley relay. He was replaced by Kaylee McKeown in the final, where Australia won the bronze medal.^[6][7]

In May 2022, Cooper competed at the Australian Championships in Adelaide. He went 24.44 in the 50 m backstroke, breaking the Australian record of 24.54 set by Ben Treffers in 2014.^[5]

In June 2022, Cooper competed at the World Championships in Budapest. He finished twelfth in the 100 m backstroke in a time of 53.55.^[8] He swam in the heats of the mixed 4 × 100 m medley relay and was replaced by McKeown in the final; Australia eventually won the silver medal.^[9] Cooper finished eighth in the 50 m backstroke in a time of 24.76.^[10] He swam the backstroke leg of the men's 4 × 100 m medley relay, splitting 54.29; Australia finished fourth in a time of 3:31.81.^[11]

In December 2022, Cooper competed at the World Championships (25 m) in Melbourne. His first event was the 100 m backstroke, where he won the bronze medal in a personal best time of 49.52.^[12] He then competed in the 50 m backstroke. He went 22.79 in the heats, breaking the Australian record of 22.91 set by Mitch Larkin in 2015. In the semifinals, Cooper lowered the mark to 22.52, also breaking the world junior record.^[13] He then swam in the men's 4 × 50 m freestyle relay, splitting 21.25 on the first leg. Australia won the gold medal in an Oceanian record time of 1:23.44.^[14] The following day, Cooper swam in the 50 m backstroke final, which he appeared to win in a time of 22.49, however, due to a technical error, the results were not made official. The final was rescheduled; Cooper ultimately won the silver medal in a time of 22.73.^[15] He then swam the backstroke leg of the 4 × 50 m medley relay, splitting 22.66; Australia won the bronze medal in a time of 1:30.81.^[16] In his final event of the competition, Cooper swam the backstroke leg of the 4 × 100 m medley relay, recording a personal best time of 49.46. Australia tied with the US for the gold medal, recording a time of 3:18.98 to break Russia's world record of 3:19.16 from 2009.^[17]

2023–2026

In March 2023, Cooper competed at the New South Wales Championships in Sydney, where he went 24.38 to set a new Australian record in the 50 m backstroke.^[10]

In July 2023, Cooper competed at the World Championships in Fukuoka. His first event was the 100 m backstroke, where he finished seventeenth in a time of 53.95.^[18] He then competed in the 50 m freestyle. He finished fourth in the final in a time of 21.70.^[19] He then finished ninth in the 50 m backstroke in a time of 24.86.^[20]

In February 2024, Cooper competed at the World Championships in Doha. He finished fourth in the 50 m butterfly in a time of 23.12.^[21] He then swam in the 50 m freestyle, finishing fifth in a time of 21.77.^[22] His final event was the 50 m backstroke. He set an Australian record of 24.12 in the semifinals, then in the final, won the gold medal in a time of 24.13.^[23]

In July 2024, Cooper competed at the Olympics in Paris. In the 100 m backstroke, he went 54.21, tying with the Netherlands' Kai van Westering for twenty-first place.^[24]

In December 2024, Cooper competed at the World Championships (25 m) in Budapest. He finished twenty-first in the 50 m butterfly in a time of 22.66.^[25] Cooper swam in the 100 m backstroke, finishing seventh in a time of 49.60.^[26] He then swam the backstroke leg of the mixed 4 × 50 m medley relay, splitting 22.68 on the backstroke leg; Australia finished fourth in a Oceanian record time of 1:36.78.^[27] Cooper then swam in the 50 m backstroke, winning the silver medal in an Australian record time of 22.49.^[28] His final event was the 4 × 100 m medley relay, in which he split 49.81 on the backstroke leg; Australia finished sixth in a time of 3:22.03.^[29]

In August 2025, Cooper competed at the World Championships in Singapore. He finished seventh in the 50 m backstroke in a time of 24.61.^[30]

In July 2026, Cooper competed at the Commonwealth Games in Glasgow. In the 50 m backstroke, he finished sixth in a time of 24.73.^[31] He then swam in the 100 m backstroke, where in the semifinals, he went 54.58 to qualify eighth-fastest for the final; he later withdrew from the final. 4 × 100 m medley relay, in which he swam the backstroke leg. He split 55.00 in the heats and was replaced in the final; Australia eventually won the gold medal.^[32][33]

Short course metres

^a split 49.46 (backstroke leg); with Joshua Yong (breaststroke leg), Matthew Temple (butterfly leg), Kyle Chalmers (freestyle leg)