Hathaway was born in Naples, Florida, but raised in Kennebunkport, Maine, and got his hockey training in New England; one of his instructors at a summer hockey camp when he was 7 years old was longtime NHL center Ken Linseman. Hathaway played on a line with future New York Rangers first-round pick Chris Kreider at prep school, then spent four seasons at Brown University, where he had 58 points (20 goals, 38 assists) and 178 penalty minutes in 121 games.

The undrafted free agent spent his first full pro season with Adirondack of the American Hockey League in 2014-15, and impressed the Calgary Flames enough to earn a two-way contract. He spent most of the 2015-16 season with the Flames’ relocated AHL team in Stockton, California, but got his first call-up late in the season and made his NHL debut on Feb. 29, 2016.