Desgagnés made his international debut for Canada competing in the U20 race at the 2017 IAAF World Cross Country Championships in Kampala, Uganada. He placed 65th.
In June, he ran a 3000 metres steeplechase personal best of 8:49.83 at the Portland Track Festival. The following month he won the Canadian U20 in the steeple and took gold at the Pan American U20 Championships in the same event. He capped off his season with a first place finish at the 2017 Canada Summer Games.
In June, he improved his 3000 m steeplechase personal best by 10 seconds, running 8:39.03 in Portland, Oregon. On July 1, he placed second at the Canadian Championships in Ottawa.
2019
Following a double gold performance 1500 m and 3000 m at the RSEQ Championships in February, he headed to the 2019 USPORTS Championships in Winnipeg. Desgagnés would finish runner-up in both the 1500 m on March 9 and 3000 m on March 8, losing to Western's Jack Sheffar in both events. He also anchored Laval's 4 x 800 m relay team, placing 13th.
Outdoors, he once again improved he steeplechase by another 10 seconds, running 8:29.10 at the Stanford Invitational. In July, he won a bronze medal in the steeplechase at the 2019 Summer Universade and placed fourth at the Canadian Championships in Montreal.
2020
At the 2020 RSEQ Championships, Desgagnés defended his 1500 m and 3000 m conference titles. At the USPORTS Championships in Edmonton, he once again finished runner-up in the 1500 m, but claimed his first USPORTS individual titles, winning gold in the 3000 m in 8:13.97.
In August, Desgagnés set a 5 km road personal best of 13:59 in Quebec City, missing the Quebec provincial record by just 3 seconds.[2]
2021
On May 21, 2021, Jean-Simon opened his season with a 8:29.41 steeple PB to win the Trials of Miles in New York City. The following week, he ran a personal best of 8:24.40 in Portland.
2022
In January 2022, JSD broke 4-minutes in the mile for the first time, running 3:58.60 at Boston University. At the USPORTS Championships in March, he won the 1500 m title, he also contested the 1000 m, placing seventh and the 3000 m, placing second
In November, Jean-Simon competed at the Pan Am Games in Santiago, Chile. Despite falling on the last water jump, he had enough of a lead to take gold in 8:30.14, beating the second place finisher by over 6 seconds.
2024
In February, Jean-Simon ran a 3000 m personal best of 7:47.39 in Boston. Later that month, he captured both the RSEQ 1500 m and 3000 m titles. Competing in his final USPORTS Championships representing Laval, Desgagnés contested both the 1500 m and 3000 m. With 400 m to go, Desgagnés led, followed by Matthew Beaudet of McGill, however with a final 400 m of under just under 60 seconds, Desgagnés held on to take the win in 7:59.72. In the 1500 m, he placed second, just 0.76 seconds behind Max Davies of Guelph.
