In 1955-56, Rousseau led the Quebec junior league when he scored 53 goals in 44 games for the St. Jean Braves. The next two years he split between the Hull-Ottawa franchises in the OHA junior circuit and the minor Eastern Professional Hockey League.
While in junior, Rousseau contributed to the Hull-Ottawa Canadiens' Memorial Cup championship in 1958. In one of the most competitive finals in the history of the fabled Cup, Rousseau played with and against many future NHL teammates. The Canadiens and the Regina Pats were both stocked with players destined to play for le bleu, blanc et rouge. The Western Pats featured such future NHL stars as Terry Harper, Bill Hicke and Red Berenson while Rousseau's Hull-Ottawa mates included Jean-Claude Tremblay, captain Ralph Backstrom and Gilles Tremblay.
The explosive winger acquired valuable international experience while helping Canada win the silver medal at the 1960 Squaw Valley Olympics. He was able to do so because the Habs loaned him to the Canadian representatives that year, the Kitchener-Waterloo Dutchmen. The Dutchies were short on players after losing the likes of Bill Kennedy and George Gosselin to the pros, but the addition of people like Rousseau, Harry Sinden and Bob Attersley buoyed their spirits. Rousseau put in four goals in a 19-1 rout of Japan and later scored a key marker in a crucial 6-5 win over Sweden. The youngster finished with nine points in seven games and came back to Canada with greater confidence and a better-rounded game.
In 1960-61, Rousseau earned his first chance with the Habs and played steadily in a 15-game debut. He gained a full-time place on the roster the next year and scored 21 goals a fine first season that brought him the Calder Trophy ahead of runner-up Cliff Pennington of the Boston Bruins. More important, he developed into a versatile forward whose exemplary two-way play caused coach Toe Blake to deploy him on both the powerplay and penalty-killing units.
In 1963-64, he scored 25 goals, five of which came in one game versus the Detroit Red Wings on February 1, 1964. Matters improved even more for Rousseau the next season when he led the team in scoring with 30 goals and 78 points. His 48 assists tied for the league lead with teammate Jean Beliveau and Chicago star Stan Mikita. Rousseau was an important factor when the Habs came back from being down two games against Detroit in the finals to defend their Stanley Cup title. His excellence didn't go unnoticed and he was selected to the NHL Second All-Star Team at the end of the 1965-66 season.
Rousseau helped Montreal reach the Stanley Cup finals again in 1966-67, when the Canadiens lost to the Maple Leafs in Canada's Centennial year. Montreal was back on top in 1967-68 and 1968-69 with the skillful winger playing a prominent role once again. Following a 24-goal performance in 1969-70, Rousseau was deemed expendable as the Habs began to introduce a few younger players into the lineup. He was traded to Minnesota, where he spent the 1970-71 season.
In June 1971 Rousseau was sent to the Rangers, where his career was rejuvenated. Playing with Bruce MacGregor and Ted Irvine, he scored 21 goals and helped the Rangers reach the Stanley Cup finals. They lost to the powerful Boston Bruins, but it was Rousseau's third-period goal that stunned the Boston Garden crowd and forced a sixth game in Madison Square Garden. Rousseau retired from the NHL in 1974-75 with 245 career goals, four Stanley Cup rings and an Olympic silver medal to his credit.