Rowan was born in County Antrim, fifth of the ten sons of Robert Rowan, an impoverished landowner of Scottish descent. He went to school in Carrickfergus. In 1797, he was commissioned an Ensign in the 52nd Regiment of Foot. He was promoted Paymaster in 1798, Lieutenant in 1799, Captain in 1803, Brevet Major in 1809, Major in 1811, Brevet Lieutenant-Colonel in 1812, and finally Lieutenant-Colonel - all his promotions above Captain were field promotions, not purchases.