Guillaume Lefebvre was born May 7, 1981 in Amos, Quebec. The 7th round choice, 227th overall selection of the Philadelphia Flyers in the 2000 NHL Entry Draft, Lefebvre is a graduate of the Quebec Major Junior Hockey League.
The Amos, Quebec native spent three seasons in the QMJHL with stops in Shawinigan, Cape Breton, Quebec and Rouyn-Noranda before making his minor professional debut with the Flyers AHL affiliate the Philadelphia Phantoms during the 2001 AHL Playoffs.
In 2000-01 Lefebvre had a career high 67 points with Rouyn-Noranda and finished his junior career with 162, before joining the Phantoms for the 2001-02 season. Lefebvre won the Phantoms' Rookie of the Year Award after tallying 19 goals and adding 15 assists for 34 points while playing in handful of games with the Flyers.
During the 2002-03 season Lefebvre found himself the property of three different NHL franchises. He spent the majority of the first half of the season with the Philadelphia Phantoms of the AHL with a few games in Philadelphia at the NHL level. His rights were then traded to the Phoenix Coyotes, and before ever playing a game with them he was traded to the Pittsburgh Penguins. While with the Penguins Lefebvre was able to score his first NHL goal.