Forbes was born 17 March 1765. He entered the army as an ensign in the Coldstream Guards 13 June 1781, became lieutenant and captain 21 April 1786, captain and lieutenant-colonel 23 August 1793, colonel 3 May 1796, major-general 29 April 1802, lieutenant-general 27 March 1808, and general on 12 August 1819. He served in Flanders with his distinguished regiment, and was present in the battles and sieges of Raismes, Famars, Dunkirk, Lincelles, Tournay, &amp;c. He subsequently accompanied the Helder Expedition, and was present in nearly every action which took place in that campaign. He was appointed second in command of the troops in the Mediterranean in March 1808, and in the same year sailed for Sicily. He was colonel 3rd Garrison Battalion 1807–9, 94th Foot 14 April 1809, 54th Foot 23 September 1809, and 21st Foot 1 June 1816 till his death. He was appointed Knight of the Order of Saint Januarius.