The son of 1940's NHL player Mike McMahon, Mike Jr. assumed his father's position of left defense. It quickly became apparent that son was more talented than dad, particularly his excellent puckhandling, passing and skating skills. He would go onto become a star with the Guelph Biltmores of the OHA's.
McMahon turned pro in the New York Rangers system in the 1961-62 season. However the last few years of the Original Six proved to be very difficult for young defensemen to crack the NHL. Though he would get into 60 NHL games over the next 5 years, McMahon spent most of his Rangers days bouncing around the minor leagues.
Things didn't get an easier in the summer of 1966, as McMahon was traded to the two time defending Stanley Cup champion Montreal Canadiens. McMahon never had a chance to play for the Habs.
When the NHL expanded from six to 12 teams for the 1967-68 season McMahon joined the Minnesota North Stars. In what proved to be his only full season in the league, McMahon exploded offensively. In 74 games he tallied 14 goals and 47 points.
Unable to live up to his explosive season, McMahon bounced around the NHL, playing with Chicago, Detroit, and Pittsburgh in a 2 season span. The NHL expanded again in 1970, and McMahon was selected in the Expansion Draft by the Buffalo Sabres, but after just 12 games was traded again, this time to the Los Angeles Kings. The Sabres landed Eddie Shack and Dick Duff in return. McMahon meanwhile never played for the Kings. Instead he was demoted to the minor leagues and traded back to the New York Rangers for the 1971-72 season.
McMahon welcomed the creation of the World Hockey Association. In the summer of 1972, he signed with the Minnesota Fighting Saints. Not only did he return to his high scoring ways, but he achieved relative stability as he stayed in the State of Hockey for 3 seasons.