MacDermott entered the British army in the year 1808, as an officer in the 8th Regiment of Foot, and during the same year saw active service in the West Indies, when the British forces captured the Island of Martinique, after several days' fighting in the field and six weeks siege of Fort Bourbon. After this MacDermott saw some severe work during the War of 1812 between England and the United States; and in 1812, on the frontier of the State of New York was dangerously wounded in the neck, the gullet being divided. In the winter of 1814-15 he was the bearer of a despatch from Montreal to Upper Canada, announcing the termination of hostilities. His regiment was ordered to return to Europe to join in the conflict with Napoleon, after the Emperor's escape from Elba. Ten thousand troops embarked at Quebec on this service, but on the convoy reaching the English Channel, they learned that the battle of Waterloo had been fought and won seven days previously. After this MacDermott served in the United Kingdom and the Mediterranean, and found time for a tour through France, Switzerland, and Italy. At Argostoli, in the island of Cephalonia, he became acquainted with Lord Byron, who entrusted him with the three last cantos of "Don Juan", to be delivered to Sir John Cam Hobhouse, a commission which MacDermott executed, having just then obtained leave of absence in order to visit England. Subsequently he was with his regiment in various parts of the United Kingdom till the year 1829, when he resolved to sell out of the army and emigrate to Australia.