General Sloper had an impressive career in the British Army. He was commissioned into the 10th Dragoons before being promoted to Major in 1755. In 1759 he was made Commanding Officer of the 1st King's Dragoon Guards. He served during the Seven Years War and was praised by the Duke of Brunswick and other senior generals for his leadership. In 1778 he served in Ireland and the following year he was deployed to India and served in the Second and Third Anglo-Mysore Wars. In 1785 he was appointed Acting Commander-in-Chief in Madras and, later that year, he was promoted to Commander-in-Chief of the British Army in India. In 1788 he was made a Knight Commander of the Order of the Bath. In 1796 he became General.
Sloper lived at West Woodhay House, West Woodhay, Berkshire. He married Jane Willis with whom he had 6 sons and 4 daughters. He is buried at East Woodhay, Hampshire.