 Born into a seagoing family at Whitby in Yorkshire in 1760, Moorsom was late to the navy, only joining aged 17, a good four years later than most of his contemporaries.He made up for his late arrival with exhaustive service, which included action at the Great Siege of Gibraltar and at the battle of Cape Spartel.He was made lieutenant rapidly in 1784 following commendations from an action against a French convoy to America two years before, and then profited by the buildup to war in 1790 when he was granted promotion to Post captain.