He was born in Dublin and educated at Thomas Sheridan's school and then at Trinity College, where he became Professor of Oratory in 1763. He translated the Orations of Demosthenes in three volumes and wrote a life of Philip of Macedon in 1758. He wrote an influential "History of Ireland from the Invasion of Henry II" in 1773. His portrait, by John Dean, is held by the National Portrait Gallery.
Leland was made a fellow of Trinity College Dublin in 1746. He was ordained a Church of Ireland priest in 1748, he served as vicar in Bray, Co. Wicklow, in 1773 he was appointed Vicar of St. Ann's Church, Dawson Street, in Dublin.