[8] Barzal's first NHL point came on October 15, 2017, against the Los Angeles Kings when he assisted on a goal by Josh Bailey.

[9] Barzal's first multi-point NHL game came on October 26, 2017, when he had a goal and an assist playing against the Minnesota Wild.

[11] On January 13, 2018, in a 7–2 win over the Rangers, he became just the fifth player in NHL history to have two five-point games in a season before his 21st birthday.

Following the departure of Tavares during the 2018 off-season, Barzal began the 2018–19 season centering the Islanders' top line with Anthony Beauvillier and Josh Bailey.

The Islanders lost to eventual Stanley Cup champion Tampa Bay Lightning in six games in the Eastern Conference Finals.

[25][26] On January 1, 2023, in his 400th NHL regular season game, Barzal scored his 100th career goal in a 4–1 loss to the Seattle Kraken.

[27] Barzal helped Canada capture the bronze medal at the 2014 IIHF World U18 Championships[28] and won gold at the 2014 Ivan Hlinka Memorial Tournament.

[29] Barzal was again named to the Canada roster the following year to compete at the 2015 IIHF World U18 Championship, where he again won bronze.