He was drafted 5th overall by the Toronto Maple Leafs in the 2012 NHL Entry Draft. Rielly was able to keep a spot on the Maple Leafs roster at the start of the 2013–14 NHL season. On October 5, 2013, Rielly made his NHL debut in a 5–4 shootout win against the Ottawa Senators. On December 16, 2013, he scored his first NHL goal against Marc-André Fleury of the Pittsburgh Penguins in a 3–1 loss.