As commanding officer of the 89th Regiment of Foot, 'Blayney's Bloodhounds' as they were called, he fought with distinction in the Napoleonic Wars.

He wrote a two-volume account of his experiences in the Napoleonic Wars - Narrative of a Forced Journey through Spain and France as a Prisoner of War in the Years 1810 to 1814, by Major-General Lord Blayney (London, 1814).

[1] He was captured by one of the O'Callaghans of Cullaville, a colonel in the French army and a prominent United Irishman who escaped after 1798.

It is said he insisted on Blayney being held to ransom for some of the United Irishmen who were in British prisons.

During Blayney's long incarceration, the 2nd Earl of Caledon looked after his financial, domestic, and political affairs, and on his return, Blayney was given a seat in parliament for Caledon's infamous "rotten borough" of Old Sarum, Wiltshire.