2007-08: Chosen in the fourth round, 70th overall, of the 2008 QMJHL Entry Draft by the Cape Breton Screaming Eagles.
2008-09: Le Sieur played in 22 games for Cape Breton, scoring 1 goal and adding 1 assist for 2 points. In 9 playoff games, Le Sieur registered no points.
2009-10: Le Sieur began the season with Cape Breton, appearing in 36 games for that club. He scored 4 goals and added 6 assists for 10 points. Le Sieur was dealt to the Shawinigan Cataractes, playing in 23 games for that club. He scored 2 goals and added 2 assists for 4 points. In 6 playoff games, he scored 1 goal and added 2 assists for 3 points.
2010-11: In 68 games for the Cataractes, Le Sieur scored 16 goals and added 28 assists for 44 points. In 12 playoff games, he scored 3 goals and added 2 assists for 5 points.