He played Gaelic football with his local club Listowel Emmets and was a member of the senior Kerry county team from the 1958 until 1962.
For 58 years, from 1962 until 2020, McMahon held the record for scoring the fastest goal in the history of All-Ireland SFC finals, until Dean Rock knocked more than 20 seconds off it.
In 1954, he helped St Michael's College to win the Dunloe Cup for the first time, which brought him to the attention of the Kerry minor team.
[3] McMahon played with the Kerry senior football team between 1959 and 1962, winning two All-Ireland medals in that time.
His debut was against Cork in the 1959 Munster Senior Football Championship final, where he scored 2-2 in the first half, led match commentator Michael O'Hehir to say: "This morning, Garry McMahon was a son of a famous father, this evening Bryan McMahon is the father of a famous son".