He was selected in the fourth round, 98th overall, by the Columbus Blue Jackets in the 2011 NHL Entry Draft and has previously played for the Minnesota Wild, Montreal Canadiens, Ottawa Senators, Boston Bruins and Florida Panthers.

Reilly proved himself to be an offensive weapon with the Golden Gophers, particularly in his sophomore and junior seasons, where he scored 33 and 42 points, respectively.

In his sophomore year, Reilly was part of a Minnesota team that reached the finals in the 2014 NCAA tournament, where they were upset by the cinderella story Union Dutchmen.

Unfortunately, Reilly would not find the stability at the NHL level that he did in his rookie year, bouncing between Minnesota and Iowa, and playing 17 games while registering a lone goal.

On February 26, 2018, Minnesota traded Reilly to the Montreal Canadiens in exchange for a fifth-round pick in the 2019 NHL Entry Draft.

However, the Canadiens would finish well outside the playoff picture for the season, leaving Reilly without a postseason appearance three years into his career.

[9] However, lineup crunches once again worked against Reilly's favor, as he found himself occasionally being scratched as the season went on, including his homecoming back to Minnesota.

Reilly was a consistent scratch for the team, playing in only 14 games before he was traded to the Ottawa Senators in early January.

As a pending free agent on a struggling Senators team, Reilly's success had him as a prime trade target approaching the deadline.

Reilly was traded to the Boston Bruins in exchange for a third-round pick in the 2022 NHL Entry Draft on April 11, 2021.

This would help set the Bruins up with a first round matchup against the Washington Capitals, allowing Reilly to get his first taste of Stanley Cup Playoff hockey.

On February 24, 2023, Reilly scored his first professional hat-trick in a 6–5 win for Providence, adding two assists as well, resulting in a five-point night.

[16] Having completed the season, on June 30, 2023, Reilly was placed on unconditional waivers for the purpose of a buyout from the remaining year of his contract with the Bruins.