Robert Monckton was the second son of Elizabeth Manners and John Monckton (later the first Viscount Galway) and, like many second sons of British aristocrats, he entered military service. In 1741, at the age of 15, he secured a commission in the 3rd Regiment of Foot Guards. He saw action in the War of the Austrian Succession, later staying on in Flanders after the bulk of the British Army had been recalled in 1745 to deal with the Jacobite Rebellion. He rose rapidly through the ranks, eventually becoming lieutenant colonel in command of the 47th Foot in early 1752.