14 Apr 1795 Surgeon's Mate 11th (Light) Dragoons obtained probably through the patronage of Sir William Erskine.
25 Dec 1796 Assistant Surgeon 11th (Light) Dragoons.
23 Aug 1799 Surgeon 20th (East Devonshire) Regiment of Foot. Remained with the 20th Regiment during the rest of his service. He accompanied the 20th Foot to Holland and was present at the storming of the entrenchments of Krabbendam and at both the actions fought at Egmont-op-Zee.
7 June 1800–26 June 1808 Served in Minorca, Egypt, Sicily, Naples, and Malta.
Present at the taking of Alexandria (1801). Arnott stated that Sir William Franklin, who was the PMO Malta and Sicily, thought well of his services.
Malta 1 June 1801 Arrived in Malta.
Malta Nov 1802 Embarked for England following the reduction of 20th Foot by one battalion. Placed on half-pay.
Malta 25 Apr 1803–16 May 1803 On half-pay.
Restored to the active list on 17 May 1803, following the resumption of hostilities with France.
Malta 1804 With his regiment at Malta.
4 July 1806 Served at the Battle of Maida.
Jan 1809 With his regiment in Portugal. He was present at the Battle of Vimiera and in the retreat on Corunna.
30 July 1809 Served at Walcheren where the 20th Regiment was greatly reduced by fever. The regiment required two years home service to recover.
1812 Served in the Peninsular War. Accompanied the 20th Regiment to Portugal, and served in Wellington's campaigns until the end of the war. He was present at the Battle of Vittoria, all the actions in the Pyrenees in which the 20th took part, and the battles of Nivelle, Nive, Orthes, and Toulouse.
1814–1819 Served in Ireland with his regiment until he embarked for St Helena in 1819.
1 Apr 1821–5 May 1821 On St Helena with the 20th Regiment during the exile of Napoleon Bonaparte. Attended on Napoleon during his last illness. On 14 April Napoleon presented the "Life of Marlborough", through Arnott, to the officers of the 20th Foot.
5 May 1821 Present at the death of Napoleon which occurred at 49 minutes past five in the afternoon of 5 May 1821. The emperor bequeathed to Arnott the sum of 600 napoleons.
Author: An account of the last illness of Napoleon. London 1822.
1822 Accompanied his regiment to India.
25 Dec 1826 Retired through ill health after returning from India in July 1826. He had completed 30 years service, twenty seven of which were with the 2oth Regiment.