Reid was commissioned in the Corps of Royal Engineers, Board of Ordnance, as no. 419, with the rank of second lieutenant on 10 February 1809, promoted first lieutenant on 23 April 1810, whereupon he joined Wellington's army at Lisbon. From 1810 to 1814 he served with the British army in the Peninsular War. He returned to England in 1814 and was promoted second captain on 20 December. In 1815 he served in the latter stages of the Anglo-American War including participating in Sir Edward Pakenham's unsuccessful attack on New Orleans. In 1816 he returned to Woolwich to become adjutant of the Royal Sappers and Miners and in the same year he accompanied the expedition against Algiers under Lord Exmouth. From 1819 to 1824 he was on half pay. Between 1824 and 1827 Reid served with the Ordnance Survey in Ireland then without employment until on 28 January 1829 he was promoted regimental first captain and sent to Exeter to quell the reform riots.