He was the second son of William Leslie, fifth laird of Warthill, Aberdeenshire, and his wife Anne, daughter of James Elphinstone of Glack, and great-niece of William Elphinstone, bishop of Aberdeen.
[1] On leaving the university Leslie was for a time parish schoolmaster of Chapel of Garioch, near his father's property.
There he became a Roman Catholic convert and after studying for the priesthood at the Pontifical Scots College in Rome took holy orders.
In favour with Emperor Joseph I, and a privy councillor, he was in 1718 translated to the see of Laibach, an appointment which carried with it the dignities of metropolitan of Carniola and prince of the Holy Roman Empire.
[1] In 1725, Bishop Leslie sent his portrait and his diploma from the university of Padua, along with some correspondence, home to his brother in Scotland.