In 1779, the age of sixteen, Maitland joined the 14th regiment, serving as a Marine on HMS Union at the Great Siege of Gibraltar in 1781.
He subsequently served in the West Indies on the staff of the quarter master-general, General Cuyler.
He was promoted from Ensign to Brevet Major and also served as aide-de-camp to Sir Charles Grey at the relief of Nieuport on the Dutch coast in 1793.
In 1796 Maitland was appointed secretary to General Sir Ralph Abercromby and travelled with him to the West Indies.
Coincidentally, Maitland received the surrender of St Thomas from Van Schogen, the same Dutch Governor he had captured the island from in 1801.