 He was born on 25 June 1741 at Dysart in Fife, Scotland, the son of David Beatson of Vicarsgrange.He was educated for the military profession, and on one of his title-pages describes himself as 'late of his majesty's corps of Royal Engineers'.The "Dictionary of National Biography" states it was probably as a subaltern in this corps that he accompanied the unsuccessful expedition against Rochefort in 1757 (but he was only 15 years old and he is not listed by the Corps History as being an engineer on the expedition), and was present with the force which, reaching the West Indies early in 1759, failed in the attack on Martinique, but succeeded in capturing Guadeloupe.