Before playing in the NHL, Rielly played parts of 3 seasons with the Moose Jaw Warriors of the Western Hockey League (WHL). In the 2010–11 WHL season, Rielly recorded 28 points in 65 games played. He missed most of the 2011–12 season because of a torn anterior cruciate ligament. He returned to the team during the end of their playoff run against the Edmonton Oil Kings. Even though Rielly missed most of the season, he was still able to finish ranked fifth in the North American skaters category by the Central Scouting Bureau.