As the son of NHL player Mike McMahon, Mike McMahon Jr was of course surrounded by hockey as a child and it came as no surprise that he too followed the road to professional success. And, like his father, McMahon found himself playing left defense from a very young age. At the age of 17 he played Junior C hockey with the Fort Erie Meteors of the OHA in 1958-59. He seemed to have inherited many of the skills his father possessed, including excellent puckhandling and skating skills, and was noticed by the Guelph Biltmores of the OHA's Junior A ranks.
McMahon turned pro with the Kitchener Beavers, where he played for five games to close out the 1961-62 season. From there he joined the Sudbury Wolves of the EPHL for one year. The biggest turning point of his career occurred in 1963-64, when the NHL's New York Rangers used him for 18 games. McMahon contributed one assist during his stay with the Blueshirts, who were led by such names as; Goyette, Gilbert, Hadfield and Jacques Plante in goal. He was also called up for one game in 1964-65, but got an extended stay in 1965-66 when he appeared in 41 games.
In the summer of 1966, McMahon was traded to the Montreal Canadiens, the two-time defending Stanley Cup champions. It was a less than ideal situation for McMahon, who was unable to crack a strong Habs defense and spent the year in the CPHL with the Houston Apollos.
The NHL expanded from six to 12 teams for the 1967-68 season and McMahon was traded to the Minnesota North Stars. In his only full season in the league, McMahon had the most productive year of his career. In 74 games with Minnesota, he tallied 14 goals and 47 points. He remained a regular on the roster until February, 1969 when he was part of a multi-player trade that sent him to the Chicago Blackhawks. He dressed for 20 games with the Hawks before being placed on waivers and taken by the Detroit Red Wings. McMahon literally had not finished unpacking when he arrived in Detroit when his two-game career with the Wings came to an end when he was traded to the Pittsburgh Penguins, where he played 12 games in 1969-70.
The NHL expanded again in 1970, and McMahon was selected in the Expansion Draft by the Buffalo Sabres, but after 12 games with them, he was sent packing yet again - this time to the Los Angeles Kings in a trade that saw Eddie Shack and Dick Duff move to Buffalo. However, the Kings could not find a place for McMahon on their starting roster and he was traded to the New York Rangers.
In the summer of 1972, the new rival WHA held its General Player Draft, and McMahon was selected by the Minnesota Fighting Saints. It was there that he enjoyed the three most prosperous years of his pro career, both from an on-ice standpoint and in terms of financial compensation. He also played a year in San Diego before finishing out his playing career in 1976-77 with the AHL's Springfield Indians.