He played sixteen years in the National Hockey League (NHL), spending thirteen with the New York Rangers and three with the Pittsburgh Penguins.

With a strong squad in 1959–60 that included future notable NHL players Chico Maki, Roger Crozier and Pat Stapleton, he averaged a point a game in the regular season and playoffs and racked up an average of five penalty minutes a game in the playoffs en route to the Teepees' second Memorial Cup championship.

He joined teammates Jean Ratelle and Rod Gilbert to form the famous GAG line (which stood for "goal a game").

Named the team's captain after the trade of longtime captain Bob Nevin, he became the first Ranger - and only the sixth NHL player - to score 50 goals in a season, nearly doubling his previous best marks; with his linemates Ratelle and Gilbert, the GAG Line totalled 139 goals and 325 points en route to the Stanley Cup Finals.

The Rangers signed Hadfield to a controversial and lucrative contract the following summer to deter him from defecting to the newly created World Hockey Association.

At the time of his retirement, Hadfield was fourth in Rangers' franchise history in goals, assists, and points (behind his linemates Jean Ratelle and Rod Gilbert, and Andy Bathgate), second in penalty minutes (behind Harry Howell) and fourth in games played (behind Ratelle, Howell and Gilbert).