After a trip to the 1975 Stanley Cup Finals, McNab was traded to the Boston Bruins. He enjoyed the best years of his NHL career in Boston, scoring at least 35 goals and 75 points six seasons in a row and playing in the 1977 NHL All-Star Game. He twice scored a playoff overtime winning goal. On December 23, 1979, during a game at Madison Square Garden in New York, McNab, teammate Mike Milbury, and several other Bruins climbed into the stands to confront fans. McNab engaged in a physical confrontation with one fan, and was soon joined by Milbury, who removed the fan's shoe and proceeded to strike the fan with it. On April 9, 1981, North Stars goaltender Don Beaupre stopped a McNab penalty shot, the first penalty shot ever taken by a member of the Bruins in a playoff game. Today, McNab is among Boston's top ten career leaders in goals, points and playoff scoring.