cullen was born in hamilton, lanarkshire. his father william was a lawyer retained by the duke of hamilton as factor, and his mother was elizabeth roberton of whistlebury. he studied at the old grammar school of hamilton (renamed in 1848 the hamilton academy), then, in 1726, began a general studies arts course at the university of glasgow. he began his medical training as apprentice to john paisley, a glasgow apothecary surgeon, then spent 1729 as surgeon on a merchant vessel trading between london and the west indies. after two years as assistant apothecary to mr murray of henrietta street, london, he returned to scotland in 1732 to establish himself in general medical practice in the parish of shotts, lanarkshire. from 1734 to 1736 he studied medicine at the university of edinburgh, where he became interested in chemistry, and was one of the founders of the royal medical society in his first year of study.