He was the 2016 Olympic champion in Rio de Janeiro in the 100 metre freestyle, winning the gold medal in world junior record time.

In the Men's 4x100m freestyle relay final, he delivered an anchor leg split of 46.59 seconds, the fastest of any swimmer in the race, helping his team secure the silver medal.

[6][7] Having moved to Adelaide to pursue better schooling and sporting opportunities,[8] he attended Immanuel College in South Australia.

[9] Competing as a 15-year-old at his first senior international swimming championships, the 2014 Oceania Swimming Championships in Auckland, New Zealand in May, Chalmers won the gold medal in the 50-metre butterfly with a time of 24.35 seconds, finishing three-tenths of a second ahead of silver medalist in the event Nielsen Vary of New Zealand.

[13] On the first day of the 2015 World Aquatics Championships in Kazan, Russia in early August, Chalmers split a 47.92 for the second leg of the 4×100 metre freestyle relay in the prelims heats, helping achieve a thirteenth-place finish in 3:16.34.

[29] At the 2016 Summer Olympics in Rio de Janeiro, Brazil, Chalmers became the first Australian to win the gold medal in the 100 metre freestyle since Michael Wenden in 1968, doing so in a new world junior record time of 47.58 seconds and finishing over two tenths of a second ahead of silver medalist Pieter Timmers of Belgium and bronze medalist Nathan Adrian of the United States.

In the 4×100 metre medley relay final, he had the fastest split of the field with a time of 46.72, which helped the team to win the bronze medal.

[46] On day one of swimming competition at the 2019 World Aquatics Championships in Gwangju, South Korea, Chalmers won a bronze medal in the 4×100 metre freestyle relay, splitting a 47.06 for the anchor leg in the final to help achieve a mark of 3:11.22 after splitting a 47.98 in the prelims heats to help advance the relay to the final ranking fourth in 3:12.65.

[53] Chalmers battled numerous injuries in the lead up to qualification for the Tokyo 2020 Olympics, which included undergoing surgery to his shoulder as well as his heart.

[56] Despite the troublesome preparation, he qualified fastest for the men's 200 metre freestyle in the Australian Olympic swimming trials after recording a time of 1:45.48.

[58] Chalmers then backed up that performance with a strong showing in the men's 100 metre freestyle, delivering his fastest time in two years of 47.59 seconds.

[65][66] In advance of competition, FINA also featured a picture of Chalmers with his silver medal from the 100 metre freestyle at the 2020 Summer Olympics on their website, highlighting him as one of the star athletes to compete in the World Cup circuit.

[67] The first day of competition, 1 October, Chalmers won the gold medal in the 50 metre freestyle with a time of 21.01 seconds in the final.

[92][93][94] The second day of competition, Chalmers swam his way to the final of the 100 metre freestyle with a time of 47.55 and an overall third-place ranking in the morning prelims.

[98][101] The new records broke those set by Australian Matthew Abood approximately 12 years earlier at the 2009 FINA Swimming World Cup.

[107][108] Leading up to the fourth and final stop of the World Cup circuit, held in Kazan, Russia at the Palace of Water Sports, Chalmers was noted by FINA as someone to watch in the freestyle races, especially in competition against some of the home country competitors including Vladimir Morozov and Kliment Kolesnikov.

[109] Chalmers opened fast in the prelims heats on day one of competition, 28 October, swimming a 21.15 in the 50 metre freestyle to advance to the final ranked first, fastest, overall.

[135] Day two, he ranked first in the prelims heats of the 50 metre butterfly, qualifying for the final with a personal best time of 23.42 seconds, where he won the gold medal and national title with a personal best time of 23.21 seconds and qualified for the Commonwealth Games with the other top-three finishers, William Yang and Isaac Cooper.

[134] Chalmers was named to the team Australia rosters for both the 2022 World Aquatics Championships and 2022 Commonwealth Games based on his times and place-finishes.

[138][139] In the preliminaries of the 100 metre butterfly, he placed 22nd with a time of 52.70 seconds, ranking two spots behind Youssef Ramadan of Egypt who swam a 52.42.

[140] On the second-to-last day of competition, he split a 46.98 for the second leg of the 4×100 metre mixed freestyle relay in the final to help win the gold medal with a world record time of 3:19.38.

[141][142] Splitting a 46.89 for the freestyle leg of the 4×100 metre medley relay in the final on the last day, he helped achieve a fourth-place finish in 3:31.81.

[143] The following month, Chalmers ranked first in the preliminaries of the 50 metre butterfly on the first day of swimming at the 2022 Commonwealth Games, advancing to the semifinals with a time of 23.45 seconds.

[153] Later the same month, Chalmers won the gold medal in the 200 meter freestyle at the 2022 Australian Short Course Swimming Championships with a time of 1:40.98, sharing the podium with two Americans.

[154][156] The first stop of the 2022 FINA Swimming World Cup, held beginning 21 October in Berlin, Germany, Chalmers won a silver medal in the 50 metre freestyle on day one, behind gold medalist Dylan Carter of Trinidad and Tobago, and a gold medal in the 100 metre freestyle on day two, finishing 0.50 seconds ahead of silver medalist Maxime Grousset with a 45.88.

[157][158] In his third event, he won the silver medal in the 200 metre freestyle, finishing in a time of 1:41.09, which was less than three-tenths of a second behind gold medalist Matthew Sates of South Africa.

[165] In the 200 metre freestyle on day three, he finished 0.19 seconds behind gold medalist Kieran Smith of the United States to place fourth in 1:41.97.

[166] His performances earned 152.1 points across all three World Cup stops, ranking him as the sixth overall highest-scoring male competitor for the 2022 circuit.

[173][174] The fourth day of competition, Chalmers substituted for Flynn Southam on the 4×50 metre mixed freestyle relay in the final, leading-off with a 20.97 to contribute to a silver medal-win with a time of 1:28.03, which registered as new Oceanian, Commonwealth, and Australian records.

[176][177] The following day, he contributed a 20.48 for the freestyle portion of the 4×50 metre medley relay in the final to help win the bronze medal with an Oceanian, Commonwealth, and Australian record time of 1:30.81.