His first active service was in the American War of Independence, in which he served at Lexington, Bunker Hill (where he was severely wounded) and in every engagement of Howe's army except one up to November 1778.

[1] By this time he had obtained his majority, and his next service was under Major-General Medows at St. Lucia in 1778–1779, after which his regiment served as marines in Rodney's fleet.

[n 1] After commanding the 5th in Ireland for some years, he exchanged and went with General Medows to Bombay, and served with that officer in India until 1792, taking part in various battles and engagements, notably Lord Cornwallis's attack on Seringapatam in the Third Anglo-Mysore War.

[1] He received prize money of £100,000 for the Seringapatam campaign,[2] his success establishing his reputation as a capable and experienced commander, with its political importance leading to his being offered the reward (which he declined) of an Irish peerage.

[3] In 1815 he was made a peer of the United Kingdom under the title Baron Harris, of Seringapatam and Mysore and of Belmont in the County of Kent.