Smith's family owned manors in Leicestershire and held Protestant beliefs. He became a merchant, supplying provisions to the armies of the Puritan Oliver Cromwell – during Cromwell's suppression of rebellion in Ireland — and an alderman of the City of London. His financial and landowning status was greatly enhanced by benefiting from his father's subscription to the Adventurers' Act from which he gained extensive landholdings in Ireland as a reward, and from his own speculative practice of buying additional subscriptions from other investors.