Born on the estate of Arbigland in the parish of Kirkbean, County of Kirkcudbright, Scotland, Craik was the illegitimate son of William Craik, 1703 -1798, an agricultural pioneer and landowner. He studied medicine at the University of Edinburgh, then joined the British Army after graduation and served as an army surgeon in the West Indies until 1751. Craik then opened up a private medical practice in Norfolk, Virginia, and shortly thereafter relocated to Winchester, Virginia. On 7 March 1754, Craik resumed his military career, accepting a commission as a surgeon in Colonel Joshua Fry's Virginia Provincial Regiment. While with this force, he became good friends with George Washington, at that time a lieutenant colonel in the regiment. Craik saw a great deal of action in various battles of the French and Indian War. He fought at the Battle of the Great Meadows and participated in the surrender of Fort Necessity, then accompanied General Edward Braddock on Braddock's unsuccessful attempt to recapture the region in 1755, treating Braddock's ultimately fatal wounds. Craik then served under Washington in actions in Virginia and Maryland, during various engagements with Indians.