[citation needed] In 1968, Mahon and his brother, Peter, together with another Catholic Labour MP, Walter Alldritt, threatened to resign the Whip.
[clarification needed] They had taken exception to remarks made by Douglas Houghton, Chairman of the Parliamentary Labour Party, that large families were a form of "social irresponsibility".
Only a midnight meeting with Prime Minister Harold Wilson and a written statement that Houghton's views were not party policy dissuaded the MPs from carrying out their threat.
Upon returning from his investiture Mass to the House of Commons, he was prevented from entering the Chamber by the Sergeant at Arms until he divested himself of his ceremonial sword.
In 1978, Mahon wrote a letter of protest to Ladbrokes, complaining of their "appalling taste" in opening a book on the successor to Pope Paul VI.