McKenzie's former teammate Gerry Melnyk dubbed the young player "Pieface" for his resemblance to a cartoon figure of the same name featured on the wrapper of a popular Canadian candy bar; this was later shortened to "Pie." He played junior hockey for three years with the St. Catharines Teepees of the OHA and led the league in goals and points in 1958. McKenzie made his NHL debut in 1958–59 with the Chicago Black Hawks. The following season he moved on to the Detroit Red Wings, where he lasted two years. He was then demoted again to the minors, where he played most of three seasons in the American Hockey League with the Hershey Bears and the Buffalo Bisons, and was named to the league's First All-Star Team in 1963. He returned to the NHL and the Black Hawks in 1963–64, and two years later played for the New York Rangers for part of the 1965–66 season, halfway during which he was traded to the Rangers' arch-rivals, the Boston Bruins. It was with the Bruins that the 5-foot-9-inch, 170 pound right wing had the most productive seasons of his career. He became a star in the 1967-68 season, scoring twenty-eight goals and gaining a reputation as a pesky, relentless hustler. He scored twenty-nine goals each of the next two seasons, and was named to the Second Team All-Star in 1969–70. In the playoffs that year he scored seventeen points in fourteen games, fourth on the team after Bobby Orr, Phil Esposito and John Bucyk and did so again in 1971-72. His best season was 1970–71, when he scored 31 goals and 77 points in 65 games. All in all, McKenzie scored 169 goals in his seven years in Boston and helped the Bruins win two Stanley Cup titles, in 1970 and 1972. At the end of the sixth and last game in the 1972 Stanley Cup finals, when the Bruins defeated the New York Rangers at Madison Square Garden 3-0 to take the Cup, McKenzie skated to center ice, raised one arm in mimicry of the Statue of Liberty, placed his other hand around his neck to appear as though he were choking, then jumped up and down in a circle several times. . This became known as the "McKenzie Choke Dance," or simply the "choke dance." In the summer of 1972, McKenzie was disgruntled at being left unprotected in the expansion draft, and he signed as player-coach with the Philadelphia Blazers of the newly formed World Hockey Association. In thirteen games he recorded only two wins and eleven losses, and he stepped down as coach in favor of veteran Phil Watson. He continued to play effectively for the Blazers, then for the Minnesota Fighting Saints, the Cincinnati Stingers and finally the New England Whalers. He finished his career in the WHA's final season in 1978-79, having played twenty-one seasons of professional hockey in the NHL and WHA.
His #19 was retired by the Hartford Whalers, making him one of only three players whose number was retired by an NHL franchise for which he never played.
