Mahoney began umpiring at the VFL Reserve Grade. He umpired in the 1964 VFL Reserve Grade grand finale. The following year, after the retirement of Jack Lee, Mahoney was promoted to the VFL senior list. Mahoney umpired in the semi-finals of the
VFL in 1967, however it was not until 1971 that he officiated in a VFL grand finale. He became a VFL Umpires Association life member in 1974. In 1981, having reached the then mandatory retirement age of 50, he ended his on-field career. The 1981 VFL grand final was a fitting end to a career believed at the time to be a record for goal umpiring at 289 matches. ==Coaching career==