Hagel's NHL career took off after he was traded to the Tampa Bay Lightning by the Chicago Blackhawks on March 18, 2022. He helped the Lightning reach the 2022 Stanley Cup Final, their quest to win the Stanley Cup for the third consecutive season ended with a six-game loss to the Colorado Avalanche. He scored 30 goals in his first full season with Tampa Bay and 26 the following season.

Hagel signed an eight-year, $52 million contract ($6.5 million average annual value) with the Lightning on Aug. 22, 2023, and was named the NHL's First Star for the week ending Feb. 9, 2025, after leading the League with eight points (four goals, four assists), all at even strength, en route to the Four Nations Face-Off representing Canada. He finished the 2024-25 season with new NHL career highs in goals (30), assists (55), points (90) and plus-minus (plus-33).

Hagel helped Team Canada win silver at the Olympic Winter Games Milano Cortina 2026. It's a life he wasn't sure he'd ever have in hockey after being selected by the Buffalo Sabres in the sixth round (No. 159) of the 2016 NHL Draft. The Sabres lost their rights to Hagel after not signing him prior to the 2018 NHL Draft. When Hagel went back to the Red Deer of the Western Hockey League for the 2018-19 season, he made a deal with himself:

Give it until Christmas, and if he couldn't get an NHL contract, he'd go to school to become a teacher.

Hagel persevered and signed with the Chicago Blackhawks as a free agent Oct. 30, 2018, He then completed his fourth and final season with Red Deer, scoring 41 goals and 102 points in 66 games. He set the team record for assists with 176 before turning pro with Rockford of the American Hockey League after the WHL playoffs.

He began the 2019-20 season with Rockford and had 31 points (19 goals, 12 assists) before being called up by the Blackhawks and making his NHL debut against the San Jose Sharks on March 11, 2020.