Smith played junior hockey with St. Michael's College School in Toronto, Ontario. Upon graduating, he played three seasons of NCAA Division I collegiate hockey with the Miami University's RedHawks in the Central Collegiate Hockey Association, scoring 122 points in 121 games during his tenure there. During his sophomore and junior season, he played mostly on the team's first line.
Professional
On March 25, 2012, Smith relinquished his final year at Miami to year to sign a three-year, entry-level contract with the Dallas Stars. He played three games for the Stars to finish the 2011–12 season. During the 2012–13 NHL lockout, Smith played for Dallas' American Hockey League affiliate, the Texas Stars. He was promoted to Dallas for the season once play resumed in January 2013 and scored his first career NHL goal on February 15, 2013, against Cory Schneider of the Vancouver Canucks. In July 2013, Smith was included in a blockbuster trade between Dallas and the Boston Bruins, which sent him, Loui Eriksson, Joe Morrow and Matt Fraser to Boston in exchange for Tyler Seguin, Rich Peverley and Ryan Button. On April 5, 2014, the New England Sports NetworkTV network named Smith the 45th annual "7th Player". The "7th Player" is awarded to the Boston Bruin who most exceeded all expectations that season, as voted by the team's fans. On July 1, 2015, Smith was traded by the Bruins, along with Marc Savard's contract, to the Florida Panthers in exchange for Jimmy Hayes. On July 3, 2016, Smith signed a five-year, $25 million contract extension with the Panthers. On June 21, 2017, Smith was traded at the 2017 NHL Expansion Draft to the Vegas Golden Knights due to salary cap issues. Both he and teammate Jonathan Marchessault were acquired by the Golden Knights, with the Panthers receiving a fourth-round pick in the 2018 NHL Entry Draft. During his tenure in Florida, Smith had 87 points in 162 games. Smith would score a career-high 60 points in his first year with Vegas as the Golden Knights defied expectations and advanced all the way to the Stanley Cup Finals in their inaugural season, eventually falling to the Washington Capitals. Smith scored 5 goals and 17 assists during the 2018 Stanley Cup playoffs.
Personal life
Reilly is the younger brother of current New York Rangers defenseman Brendan Smith and National Lacrosse League player Rory Smith. The three all played minor hockey in the Etobicoke neighbourhood of Mimico with the Faustina Sports Club, then all played for the St. Michael's Buzzers minor juniors. Smith faced his brother Brendan in the Eastern Conference Quarter-finals of the 2014 playoffs when his Bruins faced Brendan's Detroit Red Wings; the Bruins ultimately won the series four games to one.
