Nothing kept Hagens from finding ice time whenever he had the urge to stickhandle or shoot at the crack of dawn. That meant setting off alarms as a 14-year-old after sneaking into Adelard Arena on the campus of Mount St. Charles Academy in Woonsocket, Rhode Island.

Hagens has been playing the game for as long as he can remember, even battling the elements on the homemade 20-by-40 outdoor rink with older brother, Mike, and younger sister, Emma. It paid off when he finished third on Boston College as a freshman with 37 points (11 goals, 26 assists) in 37 games skating mostly as the No. 1 center between left wing Gabe Perreault (New York Rangers) and Ryan Leonard (Washington Capitals). His NCAA career started with a seven-game point streak (one goal, 10 assists) from Oct. 11-Nov. 8.

Hagens was selected by the Boston Bruins with the No. 7 pick in the 2025 NHL Draft after handling himself well as a freshman despite being the fourth-youngest player in college hockey. He played with his older brother, Michael, adjusted to the defensive side of the game and learned how to become a strong two-way center under BC coach Greg Brown. The left-handed shot ranked fourth among NCAA Division I freshmen in points, was named to the Hockey East All-Rookie Team and was No. 3 on NHL Central Scouting's final ranking of North American skaters eligible for the 2025 draft.

James' father, Mike Hagens Sr., was a defenseman for three seasons (1997-2000) at SUNY-Brockport, an NCAA Division III school in New York. He coached his sons with the Long Island Royals Under-13 and Under-14 teams in 2019-20.

James played two seasons at Mount St. Charles (2020-22) and had 115 points (54 goals, 61 assists) in 54 games for the Under-15 team in 2021-22. He moved on to the USA Hockey National Team Development Program in Plymouth, Michigan, in 2022-23 and led the U-18 team with 63 assists, 102 points and an average of 1.76 points per game in 58 games in 2023-24.

The Bruins signed Hagens to an American Hockey League amateur tryout agreement with Providence on March 23, 2026, and a three-year, entry-level contract ($975,000 average annual value) on April 8. He led Hockey East that season with 47 points (23 goals, 24 assists) in 34 games for Boston College. He also led the team with six game-winning goals, two hat tricks and 133 shots on goal.

Hagens had an assist in his NHL debut, a 3-2 win at the Columbus Blue Jackets on April 12, 2026.

Internationally, Hagens had 22 points (nine goals, 13 assists) in seven games to help the United States to a silver medal at the 2024 IIHF World Under-18 Championship. It was a single-tournament record, breaking the mark set by Nikita Kucherov in 2011 (11 goals, 10 assists with Russia). He tied for the team lead with five goals and finished with nine points in seven games to help his country win a second straight gold medal at the 2025 IIHF World Junior Championship.