Gaudreau was noted as helping to grow opportunities for smaller hockey players;[4] despite ultimately measuring in at 5 ft 9 in (1.75 m), he was a seven-time NHL All-Star,[5] and finished fourth in Hart Memorial Trophy voting twice.

He had two sisters and a younger brother, Matthew, who played hockey for the Worcester Railers and the Bridgeport Sound Tigers in the ECHL and AHL, respectively.

[18][5] After attending the Calgary Flames training camp ahead of the 2011–12 season, Gaudreau started his career in the NCAA with the Division I Boston College Eagles team in the Hockey East conference.

He then led the Eagles to a fourth consecutive Beanpot championship after helping to win a gold medal for the United States at the 2013 World Juniors.

According to Gaudreau, one of the main reasons he stayed was to play with his younger brother Matthew, who joined Boston College's team in the fall.

[35] Gaudreau was selected to play in the 2015 NHL All-Star Game in Columbus, Ohio, on January 25, 2015, and participated in the Skills Competition, garnering attention with Jakub Voráček for their antics during the shootout challenge.

[40] Gaudreau was named to Team Toews and recorded two assists in the game, both on goals scored by the Nashville Predators' Filip Forsberg.

[44][45] He was a finalist for the Calder Memorial Trophy as the NHL's best rookie, but the award went to Florida Panthers defenseman Aaron Ekblad, with Gaudreau finishing third.

Gaudreau was selected as a finalist to be the All-Star Game MVP, an honor eventually given to unlikely write-in candidate enforcer John Scott.

[58] He was named an NHL All-Star for the fourth consecutive year, but would not participate in the playoffs as the Flames came up short of a late-season push for the last wild card spot in the Western Conference.

[60] Gaudreau finished fourth in voting for the Hart Memorial Trophy, awarded by the Professional Hockey Writers' Association (PHWA) to the league's most valuable player.

The Flames had been eighth in the Western Conference when the regular season was suspended, and so played the ninth-place Winnipeg Jets in a special qualifying round.

[66] The year began slowly for Gaudreau, and further difficulties arose when coach Geoff Ward was replaced midway through the season by Darryl Sutter.

The Flames did not qualify for the 2021 Stanley Cup playoffs, coming four points short of the Montreal Canadiens for the final berth in the North Division.

With centreman Elias Lindholm and Matthew Tkachuk on the right wing, he formed one of the most dominant forward lines in the NHL, and all three members hit numerous personal and collective milestones throughout the season.

[70] Gaudreau's performance through the season led many to argue that he should be a serious candidate for the Hart Memorial Trophy, though he was ultimately not a finalist, finishing fourth in the voting.

[79] With the pending expiration of Gaudreau's contract, many regarded him as the top unrestricted free agent of 2022, and whether the Flames could re-sign him was a topic of considerable discussion.

[88] The Blue Jackets ultimately finished thirty-first of thirty-two teams in the league, while Gaudreau had a team-leading 74 points, 22 more than second-place Patrik Laine.

The Hockey News attributed the down year in part to his lack of consistent linemates,[91] as the Blue Jackets finished tied with Montreal both for the worst standings in the Eastern Conference and fourth-worst overall.

[97] He scored one goal and eight assists during the tournament en route to a bronze medal for the U.S.[17][98] On April 19, 2019, Gaudreau was selected to represent Team USA at the 2019 IIHF World Championship, held in Bratislava and Košice, Slovakia.

[100] After several years away from international play, Gaudreau was named to the U.S.' roster for the 2024 IIHF World Championship in Prague and Ostrava, Czechia.

[101] He scored three goals and eight assists during the tournament,[102] but the United States once again failed to medal, finishing in fifth after a quarterfinals loss to the host Czechs.

He previously played for the Bridgeport Sound Tigers of the American Hockey League (AHL) and their ECHL affiliate, the Worcester Railers.

[110] On the evening of August 29, 2024, Gaudreau and his brother Matthew were struck and killed by an alleged drunk driver while they were cycling in Oldmans Township, New Jersey.

[111] The driver, 44-year-old Sean M. Higgins of Woodstown, was arrested for suspected drunk driving and charged with death by auto, after telling responding officers at the scene that he had consumed "five or six" beers before driving and continued to consume alcohol while operating the car; he failed a breathalyzer test when administered by police, along with failing a field sobriety test.

[118] All 32 NHL teams issued statements in response to his death, alongside dozens of hockey players, athletes from other sports, and politicians, among them Canadian Prime Minister Justin Trudeau, Canadian Opposition Leader Pierre Poilievre, New Jersey Governor Phil Murphy, and former governor Chris Christie.

[122] Similarly, the Calgary Stampeders held a 13-second moment of silence prior to their Canadian Football League (CFL) game on September 2.

[123] On September 3, fellow American NHLer and U.S. national teammate Cole Caufield announced that he would be switching his jersey number from 22 to 13 beginning in the 2024–25 season in honor of Gaudreau, citing him as an inspiration early into his hockey career owing to his own short stature.

[124][4] Both the Blue Jackets and Flames held candlelight vigils at their respective arenas on September 4; the former saw speeches from multiple players, including Blue Jackets captain Boone Jenner, Calgary and Columbus teammate Erik Gudbranson, and Cole Sillinger, with those gathered observing 13 minutes and 21 seconds of silence, in reference to Johnny's jersey no.

[131] The defense filed a motion to dismiss the charges on February 5, 2025, in part based on "failure to present a prima facie case.