Lajeunesse played four seasons in the QMJHL with stops with the Moncton Wildcats, Acadie-Bathurst Titan and Val d'Or Foreurs. He won the Jacques Plante Memorial Trophy in 1999-00 after sporting a 31-15-1 record with six shutouts and 2.61 goals against average.
A goaltender who anticipates the play well and has strong positioning in his net, Lajeunesse spent the majority of the 2001-02 season with the Senators AHL affiliate the Grand Rapids Griffins, 13 games with the ECHL's Mobile Mysticks and one game with the NHL club.
He won the Harry "Hap" Holmes Memorial Award (AHL top goalie) in 2001-02.
He retired in 2005.