Drafted by the Toronto Maple Leafs in the third round of the 1963 NHL Amateur Draft (17th overall), McKenny had difficulty staying in the NHL early in his career and often played forward instead of his usual position on defence. He was called up from the Marlies to play two games with the Leafs in the 1965–66 season. He was given two other opportunities to make the Leafs teams in the 1966–67 and 1967-68 seasons but only played a total of eleven games. However, on Feb. 24, 1968, he scored the game-winning goal in a 1-0 win over Boston, which was the second goal of his NHL career. He struggled to stick with the Maple Leafs for several years, attributed to a poor attitude, possibly his alcoholism and his antipathy towards the high-pressure style of coach and general manager Punch Imlach. He played in the minor leagues for the Tulsa Oilers, the Rochester Americans, and the Vancouver Canucks of the Western Hockey League. McKenny and Don Cherry were roommates when on the road with the Americans.