He entered the army as a cornet in the 23rd Light Dragoons on 1 January 1797, and was present at the Battle of Ballinamuck in the Irish rebellion on 8 September of the following year. He subsequently served with his regiment at the capture of the Cape of Good Hope in 1806, and in South America as "aide-de-camp" to General (afterwards Lord) William Beresford, with whom and the rest of the troops under General Beresford's command he was made a prisoner of war, and remained a prisoner for 18 months, until released under the convention made by General Whitelock. On his return from America, Arbuthnot, then a captain in the 20th Light Dragoons, resumed his position on General Beresford's staff at Madeira, and served with him as "aide-de-camp", and afterwards as military secretary, throughout the greater part of the Peninsular War.