[5][6][7] McIntosh first drew recognition when, at age 14, she was the youngest member of the Canadian team for the 2020 Summer Olympics, where she achieved a fourth-place finish in the 400 metre freestyle.

"[10][11][12] In March and April 2023, in the span of five days, she set her first and second world records, in the 400 metre freestyle and 400 individual medley events, at the Canadian national trials.

[31][32] On March 4, 2022, McIntosh swam the 400 metre individual medley at a preparatory event for the Canadian swimming trials, recording a time of 4:29.12.

[34] McIntosh made her senior FINA World Aquatics Championships debut at the 2022 edition in Budapest, Hungary, with her first event being the 400 metre freestyle.

[39] Heavily favoured in the 400 m medley, she won gold on the first day of the competition schedule, improving her world junior, Commonwealth, and national records to 4:29.01.

[40] McIntosh was then given the novel opportunity to participate in Canada's 4 × 100 metre freestyle relay team, with mainstay members like Oleksiak, Sanchez and Taylor Ruck absent, winning a bronze medal.

"[41] The next day she took her more customary place on the 4 × 200 metre freestyle relay team, swimming the leadoff leg and helping take the silver medal.

[46][47] Following the conclusion of the Commonwealth Games, Swimming World magazine, assessing her "vast talent on display at two championship-level events", opined "it's not hype and bluster anymore.

Based purely on results from this year, not career medal totals or performance over a long stretch of time, McIntosh is the third-best female swimmer in the world.

[58][59] The following day, she won the silver medal in the 200 metre backstroke with a personal best time of 2:07.15, which was 1.87 seconds behind gold medallist Regan Smith of the United States.

[61] McIntosh drew headlines early in 2023 with performances at the 2023 Pro Swim Series event in Fort Lauderdale, first lowering her national and world junior records in the 200 metre butterfly.

"[65][13] Days later, McIntosh won the 400 metre medley with a time of 4:25.87, breaking the world record that Katinka Hosszú had set at the 2016 Summer Olympics.

[66] McIntosh's first event of the 2023 World Aquatics Championships was a highly-anticipated 400 metre freestyle, touted as a three-way contest between her, Titmus and Ledecky.

[68] McIntosh competed in the 200 m freestyle at the World Championships for the first time, finishing second in the semi-finals, 0.03 back of Titmus and 0.24 ahead of Mollie O'Callaghan.

[70] On July 27, McIntosh successfully defended her title in the 200 m butterfly and improved on her world junior record in the event, claiming that she "was just trying to have as much fun as possible and race as hard as I could.

The team, depleted of some of its most important members from years prior, finished in fifth, but McIntosh's 1:53.97 was the second-fastest in the event, behind Titmus, and the ninth-fastest of all time to that point.

[76] She went on to defend her gold medal in the 400 m individual medley, defeating Israeli silver medallist Anastasia Gorbenko by almost eight seconds.

[78][79] As with most of the Canadian team's top swimmers, McIntosh opted not to attend the 2024 World Aquatics Championships in Doha, citing its proximity to the 2024 Summer Olympics.

Her 8:11.39 time broke a ten-year-old national record previously set by Brittany MacLean at the 2014 Pan Pacific Swimming Championships.

[84] McIntosh was the centre of attention at Canada's Olympic swimming trials, which featured audiences, unlike in the pandemic-afflicted 2021 events.

[87] McIntosh drew headlines on the fourth day in the 400 m individual medley, where she broke her own world record with a 4:24.38, an improvement of a second and a half.

After coming third in the heats, she won the gold medal, finishing more than five seconds ahead of American silver medallist Katie Grimes to take her first Olympic title.

[93] Competing next in the 200 m butterfly, forty years after her mother's appearance in the same event in 1984, McIntosh won the gold medal and set a new Olympic record time of 2:03.03.

[101] On the first day of the event, McIntosh swam to gold in the 400 m freestyle, setting a new short course world record of 3:50.25 and lowering it by over a second in the process.

[102] She won another gold medal in the 200 m butterfly, setting a second short course world record by breaking Spaniard Mireia Belmonte's decade-old best time.

[6] On the penultimate day of the championships she won gold in the 400 m individual medley, breaking the last of Belmonte's world records and improving on it by over three seconds for a time of 4:15.48.

[103] McIntosh expressed disappointment with the result, but added that it was "a great motivator the next time I'm training and I'm hurting and I just remember what it's like to get silver, so it keeps me pushing forward.

[106] She was named to the Time 100 Next list, with a tribute written by Prime Minister Justin Trudeau, who described her as "a superstar at age 18 and still warming up.