becoming a captain in 1793, donkin saw active service in the west indies in the next year, gaining promotion to major in 1796. at the age of twenty-five he became a lieutenant-colonel and in 1798 led a light battalion with distinction in popham's expedition to ostend. he served with cathcart in denmark in 1807 and two years later won command of a brigade of three regiments in the army in portugal, which he led in victory at the second battle of porto (may 1809).