Blayney was a dedicated soldier and served in Gibraltar, the West Indies, Flanders, Ireland, Malta, Minorca and Egypt during the Revolutionary and Napoleonic wars. He was at the capture of the Cape of Good Hope in January 1806 and took part in Popham’s expedition to Buenos Aires later in the year. Despite his absence from England, his father-in-law Lord Caledon felt obliged to seat him for his borough of Old Sarum, unless he could be provided with advancement in some other way.
In 1809 Blayney applied to be postmaster in Ireland, but the viceroy ruled him out because he was due to be sent abroad. He proceeded to Cadiz in 1810 and was made a prisoner of war until 1814.
