He is the son of short story writer Bryan MacMahon, and a native of Listowel, County Kerry.

He returned to Ireland in 1967 to take up a post as a Statutory Lecturer in the Law Faculty, University College Cork (UCC).

A noted and often cited scholar, he is deemed to be an authority on the bench in relation to tortious matters in Ireland.

McMahon is also Chair of the Irish Universities Quality Board and the National Archives Advisory Council.

He subsequently served as Chairman of the Referendum Commission on Judges' pay which made recommendations to Government regarding the 29th Amendment to the Constitution.