Desnoyers established himself as the No. 1 center on the No. 1 team in the Quebec Maritimes Junior Hockey League in 2024-25, quite an accomplishment for an 18-year-old looking to make an impression in his NHL Draft-eligible season. He led Moncton and was tied for fifth in the league with 84 points (35 goals, 49 assists) despite missing eight games because of a nagging wrist injury. He led the league with 11 game-winning goals, was second in points per game (1.50), tied for third in plus/minus (plus-51) and first in face-off percentage (63.7 percent; minimum 10 face-offs).

Desnoyers was selected by the Utah Mammoth with the No. 4 pick in the 2025 NHL Draft. He won the 2025 Paul-Dumont Trophy as the QMJHL's personality of the year and the Guy Lafleur Trophy as the most valuable player of the QMJHL playoffs after he had 30 points (nine goals, 21 assists) in 19 games to help Moncton win its third Gilles-Courteau Trophy as league champion. He then had two points (one goal, one assist) in four games in the Memorial Cup.

Desnoyers was named the 39th captain in team history for the 2025-26 season during Moncton's awards ceremony June 2, his two-way reliability and infectious personality conjuring memories of past NHL captains Patrice Bergeron and Jonathan Toews. He was named the 39th captain in team history for the 2025-26 season during Moncton's awards ceremony June 2, his two-way reliability and infectious personality conjuring memories of past NHL captains Patrice Bergeron and Jonathan Toews.

Desnoyers signed a three-year, entry-level contract with Utah on March 23, 2026, after he had 78 points (22 goals, 56 assists) and a plus-36 rating in 45 games for Moncton. He was second among all QMJHL skaters in points per game (1.73), tied for fourth in assists and ranked fifth in power-play assists (26).

He is the youngest of three brothers. Elliot Desnoyers was selected by the Philadelphia Flyers in the fifth round (No. 135) of the 2020 NHL Draft.