 The youngest son of George Leslie D.D., (Rector of Clones & Kilmore) and Margaret Montgomery (sister of Colonel Alexander Montgomery (1686–1729) of Convoy House, County Donegal and Ballyconnell House, County Cavan).Alexander Montgomery (1686–1729) died in 1729 and left the Ballyconnell estate of about 4,000 statute acres to his nephew George Leslie who then assumed the name George Leslie Montgomery.He was in the inner circle of the Irish Parliament but his low-church Northern background clung to him and whilst he became a general of volunteers and supported Flood, he opposed any relaxation of the laws against Catholics.