Mahon started his career with
North Shields and Loughborough United before joining
Port Vale as an amateur in May 1966. He became a regular in the first-team after
Jackie Mudie handed him his debut at
Vale Park in a 2–1 win over
Southport on 20 August 1966, before losing his place in March 1967. He was a member of
Dick Graham's team that knocked
Leeds United out of the
FA Cup in
1971. He scored seven goals in 47 appearances in
1970–71, and scored 12 goals in 49 games in
1971–72, including two in the
Watney Cup final victory over
West Bromwich Albion. Mahon finished as the
club's top scorer and Player of the Year under
Jim Smith in
1972–73 with 12 goals in 48 games. He played 158 games for the "U's" in all competitions, scoring 33 goals, before joining
Wimbledon for a £1,500 fee in December 1973. He was a member of the Wimbledon side that beat
Burnley in the third round of the FA Cup in
1975, and he scored the winning goal in their 1–0 victory. It was the first time in more than fifty years a
non-League side had beaten a top-flight side away from home in the FA Cup. ==Post-retirement==