Mahony was born in Newcastle in 1915, but was orphaned when his mother died the day after his birth and his father was killed in action at the Battle of Passchendaele in Belgium during World War I.
He was raised by relatives in Newcastle where he went to school, until he moved to Armidale to attend De La Salle College, where he won a scholarship to the University of Sydney.
[1] He joined the Crown Solicitor's Office in 1941, interrupting his public service career in 1943 to serve for eighteen months in the Australian Army.
He was involved in the administration of legislation and law reform, and represented Australia on the delegation to negotiate improvements to the Geneva Convention on the treatment of prisoners of war.
[1] Mahony was made an Officer of the Order of the British Empire (OBE) in 1972 for his work as Deputy Secretary of the Attorney-General's Department.