Before entering the WHL, Rielly played midget hockey for the Notre Dame Hounds in Wilcox, Saskatchewan.

[1] In his final season with the Hounds, Rielly was the top-scoring defenceman in the Saskatchewan Midget AAA Hockey League, with 55 points in 43 games.

Despite the injury, Rielly still finished the season ranked fifth among North American skaters by the Central Scouting Bureau.

Rielly was selected by the Toronto Maple Leafs in the first round, fifth overall, and he signed a contract with the club shortly before the 2012–13 NHL lockout.

Rielly made his professional debut at the end of the 2012–13 season with the Toronto Marlies of the American Hockey League (AHL).

[7] On December 16, 2013, Rielly scored his first NHL goal, against Stanley Cup winner Marc-André Fleury of the Pittsburgh Penguins.

Throughout the 2015–16 season, Rielly earned much praise from Maple Leafs head coach Mike Babcock, as well as other figures of management.

[10] Before the 2016–17 season, Rielly was named as one of the Maple Leafs' alternate captains, along with teammates Tyler Bozak, Leo Komarov, and Matt Hunwick.

[18] Despite pushing the Boston Bruins to seven games, the Leafs lost the series in the first round, as Rielly finished the playoffs with five points.

[20] By picking up an assist the next game, he established the longest season-opening point streak by a defenceman in franchise history.

He had an NHL-leading six assists, along with an overtime goal against the Chicago Blackhawks, during a four-game Maple Leafs win streak.

[22] On December 20, during a 6–1 Leafs win over the Florida Panthers, Rielly recorded his second four assist game of the season, becoming the first NHL defenceman that year to reach the 40-point mark.

[27] The 2022–23 regular season was considered disappointing for Rielly by his prior standards, even notwithstanding a knee injury that caused him to miss fifteen games in November and December.

Later that year, Rielly was part of Canada's gold medal winning squad at the 2011 Ivan Hlinka Memorial Tournament.

Rielly's first experience at an International Ice Hockey Federation sanctioned event was the 2011 IIHF World U18 Championships, where he recorded two goals and one assist in seven games.

[33] During the 2012–13 NHL lockout, Rielly was selected to represent Canada at the 2013 IIHF World U20 Championships, again finishing in fourth place.

[35] Team North America went 2–1–0 in the tournament, scoring ten goals and conceding eight, suffering elimination following Finland's loss to Russia.