2009-10: William Carrier appeared in three midget AAA games with the Lac Saint-Louis Lions and was selected by Cape Breton in the fourth round (61st overall) of the 2010 QMJHL Entry Draft.
2010-11: Carrier skated in 61 games for Cape Breton in his first QMJHL season and played for Canada Quebec in the 2011 U17 World Hockey Challenge. One of seven 16-year-olds to skate for the Screaming Eagles, he scored 8 goals with 4 assists and was minus-28 with 54 penalty minutes. Cape Breton reached the playoffs despite finishing last in the Maritimes Division and in four playoff games Carrier was minus-five with no points and 2 penalty minutes. In six games at the WHC he scored 2 goals with 3 assists and had 10 penalty minutes for Canada Quebec.
2011-12:Carrier had a breakout season offensively in his second year with Cape Breton, finishing second on the team with 70 points as the Screaming Eagles moved up to fifth in the Martimes Division. He scored 27 goals with 43 assists in 66 games, trailing only overage center Jonathan Brunelle in both categories, and was minus-10 with 65 penalty minutes. In Cape Breton's one-sided first round playoff series with eventual QMJHL champion Saint John he scored 3 goals with 3 assists and was minus-4 with 16 penalty minutes in four games. Carrier joined Canada for the 2012 U18 World Junior Championship and was scoreless and minus-one in seven games for the bronze medal-winning squad.
2012-13: Carrier was off to a fast start for Cape Breton in his third season and played for Team QMJHL in the Subway Series against Russia before suffering a season-ending ankle injury in December. In 34 games he scored 16 goals with 26 assists and was minus-14 with 41 penalty minutes. He was scoreless with no penalty minutes in the two games for Team QMJHL against Russia. Carrier was ranked 18th amongst North American Skaters in Central Scouting's final rankings prior to the 2013 NHL Draft. Carrier was chosen in the second round, 57th overall, of the 2013 NHL Draft by the St. Louis Blues.
An all-weather forward who struggled with injuries in his draft year, Carrier combines good size with offensive skill and tops it off with a burning desire to leave it all out on the ice every night. He isn't afraid to play a physical style, or to take punishment in the corners or in front of the net to make a play, but has fallen victim in the past to trying to do too much on the ice. With great hands and a cannon of a shot, Carrier has already started working hard to improve his skating.
Carrier was dealt by the St. Louis Blues to the Buffalo Sabres in the deal that sent Ryan Miller and Steve Ott to the Blues in February, 2014.