He was promoted to a captaincy on 13 Oct. 1784. He took a prominent part in the invasion of Mysore, conducted by Lord Cornwallis in 1791. He superintended the artillery employed in the sieges of Nandidrúg (captured 19 Oct. 1791) and Savandrúg (captured 21 Dec. 1791).
For his skill and vigour Montagu received special commendation from Lord Cornwallis. The war concluded in favour of the English in 1792. On 1 March 1794 Montagu was made lieutenant-colonel, being now third on the list of Bengal artillery officers.