He took part in the Peninsular War as aide-de-camp (ADC) to Lieutenant General Sir John Moore from 1808 to 1809.
When Napoleon returned in 1815, Major Percy served as ADC to the Duke of Wellington and was present at the battles of Quatre Bras and Waterloo.
Having been the only one of Wellington's ADCs to survive Waterloo unscathed, he was assigned the task of carrying to London the dispatch announcing victory and the two French Imperial Eagles captured in the battle.
Leaving immediately after the battle, he crossed the Channel on board the sloop HMS Peruvian, having rowed some of the way.
Arriving at Downing Street on 21 June at 10 pm he informed foreign secretary Earl Bathurst of the victory; then he delivered the dispatch and captured eagles to the Prince Regent at St James's Square.