Born in the county of Cumberland in England, Jackson was sent to Charleston in South Carolina after the death of his parents. He was raised by a family friend and prominent merchant, Owen Roberts, who was the commander of a militia battalion. After the war broke out in 1775, Roberts joined the Patriot side, and the teenaged Jackson followed; Roberts probably helped Jackson to obtain a position as a cadet in the 1st South Carolina Regiment. In May 1776 Jackson was commissioned as a second lieutenant.