2012–2013 Horton first represented Australia at the
2012 Junior Pan Pacific Swimming Championships in
Honolulu at the
Veterans Memorial Aquatic Center, where he won gold in the 1500 metre freestyle in a championship record time of 15:10.07. At the same meet, he finished second in the 400-metre freestyle and 4th in the 800-metre freestyle. He also placed fourth in the 4×200-metre freestyle relay with a final time of 7:27.90, tenth in the preliminaries of the 200-metre freestyle with a 1:51.83, and twenty-first in the 100-metre freestyle with a 51.79. Two months later at the final leg of the
2012 World Cup in Singapore, Horton won the 1500 metre freestyle event in 14:54.25. At the 2013
Australian Youth Olympic Festival, Horton won gold in the 1500 metre freestyle event. Eight months later at the
2013 World Junior Championships in Dubai, Horton won five gold medals and a silver. He took out the 200 m, 400 m, 800 m and 1500 m freestyle events and alongside
Luke Percy,
Regan Leong and
Blake Jones won the 4 × 100 m freestyle relay all in new
Championships record times. The team of Horton, Leong,
Isaac Jones and
Jack McLoughlin finished second behind the British in the 4 × 200 m freestyle relay event.
2014–2016 Horton qualified for his first senior team at the
2014 Australian Swimming Championships where he won the 1500 metre freestyle in 14:51.55 and finished second behind
David McKeon in the 400-metre freestyle in 3:44.60, setting two new
junior world records. Horton also finished 5th in the 200-metre freestyle in 1:47.36 which also set a new junior world record. At the
2014 Commonwealth Games in Glasgow, Horton won the silver medal in the
1500 metre freestyle in new junior world record time of 14:48.76 and narrowly missed the podium in the
400-metre freestyle finishing in fourth place in 3:44.91. In the
4 × 200-metre freestyle relay event, Horton alongside
Thomas Fraser-Holmes,
David McKeon and
Ned McKendry finished as the fastest qualifies with Horton swimming the anchor leg in 1:49.17. In the final, Horton was replaced by
Cameron McEvoy and they went on to win the gold in a new
games record time of 7:07.38. Three weeks later at the
2014 Pan Pacific Swimming Championships in
Gold Coast, Australia, Horton won the silver medal in the
800-metre freestyle in 7.47.73, the bronze medal in the
1500 metre freestyle in 14:52.78 and with McKeon, McEvoy and Fraser-Holmes won bronze in the
4 × 200-metre freestyle relay in 7:08.55. In April 2014, Horton became an ambassador for Horton's Heroes Water Polo team (SHWP) with the swimwear brand
Speedo and in August 2014, after his breakthrough performances he was named the winner of the Georgina Hope Foundation Rising Star of the Australian Swim Team. At the
2016 Summer Olympics, Horton represented Australia in the 400 m freestyle, in which he won gold, and the
4 × 200 metre freestyle relay, in which he finished 4th with his teammates. He finished 5th in the final of the 1500 m freestyle.
2019–2021 At the
2019 World Aquatics Championships, Horton won silver in the
400 m freestyle event where he came runner up to
Sun Yang. In a controversial "stand-off" Horton refused to shake the hand of Sun or to stand on the winners' podium. Horton had previously called Sun a "drug cheat". On 28 February 2020, Sun was issued an 8-year ban by the
Court of Arbitration for Sport (CAS) for tampering with the doping control process, with calls to reissue medals from affected events, though the CAS clarified that Sun would not be stripped of any of his medals because "doping tests performed on [Sun] shortly before and after the aborted doping control in September 2018 were negative" and "in the absence of any evidence that [Sun] may have engaged in doping activity ... the results achieved by [Sun] in the period prior to the CAS award being issued should not be disqualified." Horton has been criticised for remaining silent on Australian swimmers who have faced punishments for violating anti-doping rules. Horton later competed in the
4 × 200 m freestyle relay. Swimming the anchor leg, he dove in with the lead and split 1:44.85, which was the fastest split in the field. Australia won the gold medal in a time of 7:00.85, which was a new Australian record, surpassing the previous mark of 7:01.65 from
2009. At the
2021 Australian Swimming Trials, Horton came third in the qualification final for the 400m freestyle, failing to qualify behind
Elijah Winnington and
Jack McLoughlin. Horton would also make it to the final for the 200m freestyle and would come sixth, earning him selection for the
2020 Olympics in the squad for the
Men's 4 × 200 m freestyle relay.
2022 The following year, Horton won the silver medal in the 400-metre freestyle, with a 3:44.06, and the bronze medal in the 200-metre freestyle, with a 1:46.70, at the
2022 Australian Swimming Championships, held in May in
Adelaide, and qualified to represent Australia at the
2022 World Aquatics Championships and in
swimming competition at the
2022 Commonwealth Games. A little over three months later, Horton qualified for and was named to the Australia roster for the
2022 World Short Course Championships, to be held in December following the Championships relocation from the
Palace of Water Sports in Kazan to his hometown of
Melbourne, based on his performances in August at the
2022 Australian Short Course Swimming Championships. Day three of the 2022 World Short Course Championships, contested at
Melbourne Sports and Aquatic Centre, he ranked fourth in the preliminaries of the
400 metre freestyle with a time of 3:38.09 and qualified for the evening final. Dropping to a 3:37.94 in the final, he placed sixth. The following day, he won a silver medal as part of the finals relay in the
4×200 metre freestyle relay, splitting a 1:43.19 for the fourth leg of the relay to help set new
Oceanian,
Commonwealth, and
Australian records in the event with finals relay teammates
Thomas Neill,
Kyle Chalmers, and
Flynn Southam in a time of 6:46.54. In the inaugural men's
800 metre freestyle at a
World Short Course Championships, the following day, he placed ninth overall with a time of 7:40.64.
2023 At the
2023 Australian Swimming Championships in April, Horton advanced to the final of the 400-metre freestyle on day one with a time of 3:54.31 in the preliminaries, then finished 0.05 seconds behind sixth-place finisher
Joshua Staples in the final to place seventh. He followed up with a placing of fifth in the 800-metre freestyle on day three in 8:08.31. He improved to a third-place finish in the 400-metre freestyle at the
2023 Australian Swimming Trials in June, finishing in a time of 3:46.71 in the final.
2024 On the 21 January 2024, Horton announced his retirement from swimming just 6 months before the 2024 Paris Olympic Games. ==International championships (50 m)==