Lajeunesse was born in Montreal, Quebec. He started playing hockey in his hometown of Montreal with the Montreal Junior Canadiens of the OHA. The young defenceman scored 22 points in combination with 172 penalty minutes in his first season with the team and helped them win the Memorial Cup. The following season in 1969–70, Lajeunesse scored 29 points and saw his penalty minutes decrease almost by half with only 87 on the season and again helped the team win their second consecutive Memorial Cup. He was proving that he could control the flow of the game with his big hits and intensity. This would prove to pay off for Lajeunesse, as the Detroit Red Wings drafted him with their first pick, 12th overall, in the 1970 NHL amateur draft.
Pro
Lajeunesse started off in Detroit's farm team, the Fort Worth Wings of the Central Hockey League. He only played in 12 games before being brought up to help the Red Wings on November 12, 1970 against the St. Louis Blues. It was hoped that his toughness and grit could make a difference with the struggling club which featured such legends as Gordie Howe and Alex Delvecchio. In the remaining 62 games of the 1970–71 NHL season, Lajeunesse tacked on 55 penalty minutes, contributed four assists, and scored what was to be his lone NHL goal. This was not what the Wings were expecting. After the Wings failed to reach the playoffs again that year, Lajeunesse was re-assessed and sent back down to the minors. The following season saw Lajeunesse bouncing back and forth in the Wings' organization. He spent time with the Tidewater Wings of the AHL and Fort Worth, while suiting up for seven games in Detroit. He contributed 20 penalty minutes in those seven games without scoring a point and finished the 1971–72 season back in Fort Worth where he helped with a short playoffrun. He continued playing in the minor leagues with Tidewater in 1972–73 before being called back to Detroit to fill in for injuries. Lajeunesse played in 28 NHL games that year and ended with one assist to go with 26 penalty minutes. The Wings had had enough and Lajeunesse was traded on May 15, 1973 to the Philadelphia Flyers for Rick Foley. He started off in the Flyers' farm system and would retire there. Lajeunesse only saw action in six more NHL games with Philadelphia in the following two seasons. He helped his AHL team the Richmond Robins reach the playoffs in three consecutive seasons, but failed to help them get past the second round. Lajeunesse retired from hockey in 1976.
