From 1950 to 1953, Mahony was educated at St Patrick's College, Silverstream, where he was proxime accessit to the dux in his final year.[1] He studied law at Victoria University of Wellington, graduating with a Bachelor of Laws degree in 1968.[2] He worked for a Wellington law firm, primarily working in the area of corporate law.[3]
In 1978, Mahony was appointed to the bench as a stipendary magistrate (now called District Court judge) based in Auckland.[1] He was one of the original appointments to the Family Court of New Zealand when it was established in 1981, and in 1985 he became the principal judge of that court.[4] He was intimately involved in work that led to the Domestic Violence Act 1995, and in 1996 he established the Family Violence Taskforce.[4] Mahony retired as principal Family Court judge in 2004, and was succeeded in that role by Peter Boshier.[5] Mahony continued as a part-time acting District Court judge until 2010.[6][7][8][9]
Following his retirement from the Family Court, Mahony served as a member of the New Zealand Parole Board, and succeeded Anand Satyanand as chair of the confidential forum for former psychiatric patients.[3]
Mahony's wife, Shirley, Lady Mahony, died on 11 August 2023.[10]