 He was appointed as Roman Catholic Bishop of Clogher in 1727 following the death of his predecessor, Hugh MacMahon on 7 August 1737.His predecessor served as Apostolic Administrator for the Diocese of Kilmore and Archbishop of Armagh.Bernard MacMahon was himself appointed to the position of Archbishop of Armagh on 8 November 1737; he died in Armagh on 27 May 1747.