Hagg, 23, was selected by the Philadelphia Flyers in the second round (No. 41) of the 2013 NHL Draft after his first season with Modo, which plays in the Swedish Hockey League second tier.

On March 21, 2014, Hagg signed a three-year, entry level contract with the Flyers and joined Adirondack of the American Hockey League, where he had four points (one goal, three assists) in 10 games. The Flyers moved their AHL team to Lehigh Valley in the 2014-15 season, and Hagg spent most of the next three seasons there. He was called up by the Flyers on April 9, 2017 and made his debut that night in Philadelphia's regular-season finale against the Carolina Hurricanes.

In 2017-18, Hagg made the Flyers out of training camp and had nine points (three goals, six assists) in 70 games. He scored his first NHL goal against the Detroit Red Wings on Dec. 21, 2017. Hagg was leading the NHL in hits (232) before he sustained a lower-body injury on March 10 and played two games during the final month of the season.