A talented left winger for most of his amateur days, Tremblay made the pragmatic switch to defense when he was notified of the shortage of blue line prospects in the Canadiens system. In his first full pro season with the Hull-Ottawa Canadiens of the Eastern Professional Hockey League in 1959-60, he accumulated 25 goals and 56 points in 55 games and his effort garnered him the league's most valuable player honors. He played 11 games for Montreal that season and the following season he was called up to the parent club for 29 games.
He experienced his first Stanley Cup triumph in 1965 when he led all post-season skaters with nine assists in Montreal's victory over Chicago. Tremblay gained prominence in the eyes of demanding Canadiens supporters for the first time. He was equally brilliant the following season when he scored 11 points in 10 playoff games to help the Habs repeat as champions against Detroit. Following the series, Tremblay narrowly lost out to Red Wings goalie Roger Crozier in the voting for the Conn Smythe Trophy.
Tremblay continued to produce offensively and helped Montreal to consecutive Stanley Cup wins in 1968 and 1969. Following the 1967-68 season, he finished runner-up in the James Norris Trophy voting to Boston superstar Bobby Orr. Nevertheless, the team was feeling the ire of the Forum crowd as their defensive play wasn't as stellar as in the past and they tended to struggle against the perceived "weak" expansion clubs.
Prior to the 1970-71 season, the Habs were deciding which of their veteran defensemen to unload as part of a rebuilding program. Tremblay's desire to play for the club was under scrutiny for the first time. A proud competitor, he voiced his desire to stay with the Canadiens. That year Tremblay broke Doug Harvey's team record for single-season points by a defenseman. He totaled a personal high of 63 points, then went on to register 17 points in 20 playoff matches to help the Canadiens win the Stanley Cup. After the playoffs, he was named to the NHL First All-Star Team for the only time in his career. Another factor that helped Tremblay was when he relinquished the distracting task of being the team's representative in the NHL Players' Association.
In 1971-72, Tremblay posted strong numbers once again with 57 points, but the New York Rangers in the quarterfinals defeated the defending champions. While negotiating a new contract with team president Sam Pollock, Tremblay was offered a deal by the WHA's Quebec Nordiques that he couldn't refuse.
During the league's seven-year existence, Tremblay was one of the biggest stars. He led the league in assists on two occasions and helped the Nordiques win the Avco Cup in 1976. Twice he was the recipient of the Dennis A. Murphy Trophy as the top defenseman in the WHA.
Tremblay was a key member of the WHA All-Star squad that represented Canada during the 1974 series against the Soviet Union. His partnership with 1972 Summit Series veteran Pat Stapleton proved to be one of the strengths of the team, and it was this set of games that finally established Tremblay as an internationally recognized star.
Tremblay retired after the announced merger of the WHA and the NHL in 1979. An interesting sidelight was that his number 3 was retired by the Nordiques for his WHA exploits. Thus, he and John McKenzie became the only two players to enjoy retired-number status with NHL clubs even though they never played with those teams when they were in the league.