His adaptation of Brendan Behan's autobiographical Borstal Boy played on Broadway after a long run in Dublin's Abbey Theatre.
After graduating from Fordham University, McMahon served as an officer in the United States Navy during World War II.
Following a successful career as an executive with NBC and MCA, McMahon moved to Ireland in 1961 where he worked with the newly established Raidió Teilifís Éireann (RTÉ).
In the mid-sixties McMahon was commissioned by Tomás Mac Anna, Artistic Director of Dublin's Abbey Theatre, to adapt Brendan Behan's 1958 book, Borstal Boy, for the stage.
McMahon also wrote Gandhi: The Fiery Gates for the Abbey, and James Connolly, a documentary for television.