After a stellar 2012–13 season with the Quebec Major Junior Hockey League (QMJHL)'s Halifax Mooseheads, in which he was named CHL Player of the Year and won the Memorial Cup, Drouin was selected in the first round, third overall, by the Tampa Bay Lightning in the 2013 NHL entry draft.

[1] On June 15, 2017, Drouin was traded to the Montreal Canadiens where he played a total of six seasons, before signing with the Avalanche as a free agent on July 1, 2023.

[5] Drouin was also a key member of Halifax's 2012 playoff success, most notably scoring the overtime winning goal in Game 7 against the Quebec Remparts.

[6] On November 7, 2012, Drouin represented Team QMJHL in Game 2 of the Subway Super Series against Russian junior players; he scored a goal and added three assists.

[13] On October 16, Drouin was assigned to Tampa Bay's American Hockey League (AHL) affiliate, the Syracuse Crunch, on a conditioning stint.

[14] However, just three days later on October 19, the Lightning recalled Drouin from his conditioning assignment with the Crunch; he registered one goal, two assists and a +4 plus-minus rating during his AHL stint.

[16] The following day, he recorded his first NHL assist on a Valtteri Filppula goal in Tampa Bay's 2–1 overtime victory over the Calgary Flames.

[22] On January 20, the Lightning announced Drouin had been suspended indefinitely without pay for a failure to report to the Crunch's game against the Toronto Marlies that day.

[23] Lightning GM Steve Yzerman held a press conference the following day, denying Walsh's claim there was a trade nearing.

[27] On April 15, Drouin recorded his first career NHL playoff assist and point on a Brian Boyle goal in a 5–2 Lightning victory over the Red Wings.

[39] The Canadiens submitted him for consideration for the King Clancy Memorial Trophy at the end of the regular season, in recognition of his charitable work.

[40] Returning to the Canadiens for the 2021–22 season, Drouin was placed on the team's second line alongside close friend Josh Anderson and newcomer Christian Dvorak.

[41] In the season-opening game against the Toronto Maple Leafs on October 13, 2021, Drouin scored the Canadiens' first goal of the season off an assist from Anderson.

[47] Drouin was a healthy scratch by head coach Martin St. Louis to begin the 2022–23 season, the final year of his contract with the Canadiens.

[50] Drouin continued to struggle with injuries, ultimately appearing in 58 games, and recording 2 goals and 27 assists for 29 points, the latter second on the team behind captain Nick Suzuki.

[54] Drouin made his Avalanche debut on October 11, 2023, registering an assist on the top-line in a 2023–24 season-opening win against the Los Angeles Kings.

[55] Initially struggling to contribute, Drouin was a healthy scratch after 8 games before returning to the lineup and notching his first goal with the Avalanche in his 12th appearance during a 5–1 victory over the Seattle Kraken on November 13, 2023.

During the final regular season contest against the Edmonton Oilers, Drouin suffered a lower body injury and was ruled out for the entirety of the first-round playoff series against the Winnipeg Jets.

As a pending free agent, Drouin opted to remain with the Avalanche by re-signing to a team friendly one-year, $2.5 million contract extension on July 1, 2024.