Son of William Caslon II, grandson of William Caslon I. He co-owned the Chiswell Street family firm from the death of his father in 1778 until 1792, when he sold his share in the foundry to his mother and his sister-in-law, the widow of his brother Henry. In the same year he purchased the Salisbury Square foundry of Joseph Jackson (apprentice to his grandfather and rival to his father), who had recently died, and called the foundry Caslon & Son. In 1807, this business was passed on to his son William Caslon IV who in turn sold up in 1819 to Blake, Garnett & Co. (later Stephenson Blake).