In 1810 Fonblanque left Cambridge due to ill-health, a burst blood-vessel on the lungs, and entered the Army obtaining a commission in the 21st Fusiliers. With this regiment he served at Cadiz, Gibraltar, in Sicily and the Greek Islands then in Italy. Lord William Bentinck, under whom he served in Italy, appointed him deputy judge advocate-general. In the American War (of 1812) he was present at the taking of Washington, at the Battle of Baltimore, and ultimately at the fatal repulse at New Orleans when he was made prisoner within the enemy's lines being one of the very few who had succeeded in crossing the works. His last service was with the army of occupation in France in 1815. He left Valenciennes in November 1816 and was almost immediately called to the bar.