After early success swimming at the university level, Masse had a breakout appearance at the 2016 Summer Olympics, winning a bronze medal in the 100 metre backstroke.

[7][8] In April 2015, Masse participated in the Canadian swimming trials, hoping to make the team for the 2015 Pan American Games that were scheduled to be held on home soil in Toronto.

[12] On the final day of swimming competition, Masse participated in her second event of the games, performing the backstroke leg for the Canadian women's team in the 4×100 m medley relay.

Expectations for the team had been somewhat raised due to the strong performances of members throughout, including both Masse and breakout star Penny Oleksiak, but they ultimately finished in fifth place.

"[4] Masse followed up her world record by participating in the final of the mixed 4×100 m medley with the lead-off backstroke leg of the race.

[17] She competed in the women's 200 m backstroke setting the Canadian record at 2:05.97 in the semis and qualifying for the final second overall, but was unable to duplicate her time and placed fifth at 2:07.04 ahead of teammate Hilary Caldwell.

[20][21] She began her part of the competition in her preferred event, the 100 m. Masse set two Commonwealth Games records en route to the final.

Following the Commonwealth Games, Masse made her Pan Pacific Swimming Championships debut at the 2018 edition in Tokyo.

Masse won the gold medal, finishing 0.11 ahead of Seebohm, though she came up short of reclaiming her world record.

[24][25] She also competed in the 200 m backstroke event, finishing in fourth place, 0.54 seconds behind bronze medallist Regan Smith.

[28] With the victory Masse became the first Canadian swimmer to defend a world title and only the second to win two golds at the event.

In June, she opted to leave the University of Toronto's swimming program to join the Canadian national team, as its facility was reopening.

[33] On November 1, 2020, during Match 6 of the ISL, Masse won the 50m backstroke and helped her team win the 4x100 medley relay.

Competing first in the 100 m backstroke event, Masse was identified as one of the three medal favourites, alongside Australia's Kaylee McKeown and American Regan Smith.

[38] For her final event, Masse was again the backstroke leg of Canada's team in the 4×100 m medley relay, facing off again against McKeown and Smith.

Masse's first event was the 100 m backstroke, where she won silver, finishing just behind Sweden's Louise Hansson while setting a Canadian record in the process.

[40] She then competed in the 200 m where she won silver behind Rhyan White, though in this distance Masse told media she was disappointed with her time though content with the medal.

[41] Masse then competed in the 50 m where she won a third medal behind teammate Maggie Mac Neil who set a world record.

[43] She won the silver medal, finishing 0.18 behind Regan Smith, standing on her third consecutive World podium but missing another title defence.

[45] She won the gold medal, her third at the World Championships, finishing 0.08 seconds ahead of American silver medallist Katharine Berkoff.

"[47] She missed the podium in the 200 m backstroke, finishing fifth, but won a bronze medal on the final day as part of the Canadian team in the 4×100 m medley relay.

[57] In what was considered a significant upset, Masse finished second in the 100 metre backstroke at the Canadian swimming trials, behind Ingrid Wilm.

[58] Masse began the 2023 World Aquatics Championships in Fukuoka competing in the 100 m backstroke, where she finished fourth, nearly a second behind bronze medalist Berkoff.

"[63] Masse recorded a fifth-place finish in the final of the 200 m backstroke, for the first time concluding a World Aquatics Championships without an individual medal.

[64] On the final day of the championships, she swam the backstroke leg for Canada in the 4×100 m medley relay, helping the team to another bronze medal.

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