MacMahon was appointed as Roman Catholic Bishop of Clogher on 17 August 1727,[2] following the death of his predecessor, Hugh MacMahon. Bernard MacMahon was transferred to the position of Archbishop of Armagh on 8 November 1737.[3] In 1741, he would go into hiding as a result of him not promoting a priest to a parish; this would result in a warrant being issued for his arrest.[4][5] He died in Armagh on 27 May 1747.[1]