Manners joined the Army as a cornet in the 3rd Dragoon Guards on 27 April 1775, and was promoted to lieutenant on 25 December 1778. On 3 October 1779 he became captain of a company in the 86th Foot, newly raised by his cousin the Duke of Rutland. He went with the 86th Foot to the West Indies, serving for a time on marine duty aboard ship before being sent with a detachment to Tobago. In 1781 the island was captured by the comte de Grasse and the garrison returned to Europe, the officers giving their parole. On 6 December 1782 Manners was promoted to major in the 80th Foot and on 19 March 1783 he was made an equerry to the King. On 14 February 1784 he succeeded Allan Maclean as lieutenant-colonel of the 1st Battalion, 84th Regiment of Foot. That regiment was reduced on 24 June 1784, and after a period on half-pay, Manners joined the 3rd Foot Guards as captain-lieutenant on 19 February 1787, still with the rank of lieutenant-colonel.