Keegan Messing was born on January 23, 1992, in Girdwood, Anchorage, Alaska, USA. He has two brothers, Paxon and Tanner; Paxon was killed in a motorcycle accident at age 26, in 2019. He holds dual U.S. and Canadian citizenship. His mother was born in Edmonton, Alberta, Canada, and he is a great-great-grandson of Manzo Nagano. His father is a firefighter.

At the 2013 U.S. Nationals, Messing placed sixteenth, followed by a twelfth-place finish the following year.

In July 2014, Messing said that he would begin competing for Canada. In the 2014–15 season, he won bronze at the Skate Canada Challenge and qualified for the 2015 Canadian Championships. He placed fifth at Nationals, representing a club in Sherwood Park, Alberta.

At the 2019 Canadian Championships, Messing was considered a favourite going in, but struggled in both programs. In the short program, he placed second behind Gogolev, despite falling on his opening quad attempt. The free skate was also a challenge, and he dropped to third place, winning the bronze medal behind Nguyen and Gogolev. He was named to the Canadian teams for the Four Continents and World Championships.

At the 2019 Four Continents Championships, Messing placed fifth in the short program after rough landings on both his triple Axel and triple Lutz jumps. He then placed third in the free program with a personal best score, winning a small bronze medal, and placing fourth overall. At the 2019 World Championships, Messing placed fifteenth after two error-riddled programs. The placements of Messing and Nguyen meant that Canada would have only one men’s berth at the 2020 World Championships. 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.