Previously, representing the United States, he was the two-time International Cup of Nice champion (2011, 2012) and the 2012 Nebelhorn Trophy bronze medalist.

[3][4] Keegan holds dual U.S. and Canadian citizenship;[5] his mother was born in Edmonton, Alberta, and he is a great-great-grandson of Manzo Nagano, the first Japanese person to officially immigrate to Canada.

[6] In the 2015–16 season, Messing placed fifth at the 2015 CS Ondrej Nepela Trophy and eleventh at the 2015 Skate Canada International, his senior Grand Prix debut.

Messing began the Olympic season at the 2017 CS Autumn Classic International, where he won the bronze medal behind Javier Fernandez and Yuzuru Hanyu.

Competing at the 2018 Canadian Championships that would decide the nation's delegation to the 2018 Winter Olympics, Messing placed third in both the short program and free skate, winning the silver medal behind Patrick Chan.

[19] At his World Championships debut, Messing placed sixth in the short program with a new personal best score, qualifying for the final flight of the free skate.

[21] Competing on the Grand Prix, Messing placed first in the short program at the 2018 Skate Canada International, following mistakes by presumed frontrunner Shoma Uno.

[23] He was initially named as the first alternate to the Grand Prix Final and was subsequently called up following the withdrawal of Yuzuru Hanyu due to injury.

He moved up to fifth place in the free skate, despite falling on a triple Axel and doubling a planned quadruple toe loop.

[30] Messing concluded the season at the 2019 World Team Trophy, where he placed sixth overall among the twelve men, including a fourth-place free skate that featured only one error.

[31] Messing selected "Perfect" as his short program music for the season in commemoration of his marriage, the song having been the first dance at his wedding.

[10] His first competition of the season was the 2019 CS Autumn Classic International, where he won the bronze medal with third-place finishes in both segments.

Messing held the Japanese flag in aid of the event's winner, Yuzuru Hanyu, during the medal ceremony and was praised in the media for an example of good sportsmanship.

[33] Messing placed third in the short program at 2019 Skate America, fractions of a point behind Dmitri Aliev, and set a new personal best.

"[36] At his second Grand Prix, the 2019 Cup of China, he was fifth in the short program after falling on his quad toe loop and performing only a double Axel instead of a triple.

[39] He struggled in the free skate, falling on both his attempted quad jumps and making a number of other errors, and dropped to third place overall behind Roman Sadovsky and Nguyen.

[42] He struggled in the free skate, making several jump errors that dropped him to eighth place overall, with Nguyen, the highest-finishing Canadian skater, at sixth.

[43] The onset of the coronavirus pandemic disrupted Messing's normal plans, including touring and spending time in Canada with choreographers and the national team.

[49] The stakes for his performance were high, as this was the primary qualifier for the 2022 Winter Olympics in Beijing and the only means for a country to earn more than one berth per discipline, which Messing would later call "quite a heavy burden.

"[55] Journeying from Alaska to Ottawa for the 2022 Canadian Championships in the midst of restrictions prompted by the Omicron variant, Messing's skates were initially lost in transit.

However, on February 1, it was reported that he had been unable to fly to China at that point due to the need to produce two negative PCR tests and that, as a result, his expected participation in the team event was especially in jeopardy.

[59] Messing was eventually cleared to fly to travel to China via Montreal and Milan, arriving on Monday, a day in advance of the men's event.

"[63] Tenth in the free skate, with his main error being tripling a planned quad toe loop, Messing finished eleventh overall.

"[64] He announced afterward that he intended to compete for one more season, hoping to attend the next edition of the World Team Trophy.

A strong free skate carried him to the gold medal, and he noted afterward that it had been "a constant battle to overcome the nerves.

[72] He was second in the free skate after a number of jump errors at the beginning of his program, but remained first overall and won his second consecutive Canadian title.

[12] Despite falling on his open quad attempt in the short program of the 2023 Four Continents Championships, Messing placed second in the segment with a season-high international score of 86.70, winning a silver small medal.

He was 5.20 points back of segment leader Kao Miura of Japan, and only 0.06 ahead of third-place American Jimmy Ma.

[78] A singled attempt at a triple Axel and an invalid spin dropped him to seventh place overall, and eleventh in the free skate, but he called it a great event "to go out on.