Born the son of a farmer near Granton, in north Edinburgh, on 18 December 1716, Cleghorn was the youngest of five children.
He began his education in the grammar school of Cramond, and entered the University of Edinburgh as a student of physic under Alexander Monro in 1731, living in his house.
[1] In 1736 Cleghorn was appointed surgeon to the 22nd Regiment of Foot, then stationed in Menorca (historically called "Minorca" by the British), and he remained on the island till it was ordered to Dublin in 1749.
[1] Successful in practice, Cleghorn in his later years spent much of his time on a small farm he owned, near Dublin.
[1] Unexplained statements in the Hippocratic writings, Cleghorn argued, become clear in the light of clinical observations on the Mediterranean coasts, and the context that diseases, both acute and chronic, are there often modified by malarial fever.