The Islanders originally claimed Reilly off waivers Florida Panthers in November 2023. Reilly re-signed with New York over the summer on a one-year deal worth $1.25 million through the 2024–25 season. Reily collected six goals and 24 points while averaging 17:15 of ice time in 59 games with the Islanders to end the year; scored a goal and an assist in five playoff contests.

The Columbus Blue Jackets originally selected Reilly in the fourth round (No. 98 overall) of the 2011 NHL Draft. The Chanhassen, Minnesota native never signed with his draft club, choosing to sign with Minnesota Wild in 2015 after spending three seasons at the University of Minnesota.

Reilly spent parts of three seasons with the Wild before being traded to the Montreal Canadiens in 2018 in exchange for a 2019 fifth-round pick. After three more seasons in Montreal, Reilly was sent to The Ottawa Senators in 2020.

Reilly split the 2020-21 season between the Senators and Boston Bruins, setting an NHL record for most points in a season by a player without scoring a single goal. He signed a three-year extension with the Bruins after the season, but was bought out after the second year of the deal.

Through 411 career NHL games with the Islanders, Wild, Canadiens, Senators, Bruins and Panthers, Reilly has collected 18 goals and 104 assists for 122 points. He added a goal and five assists for six points in 21 career playoff contests.