The 9th arrived in the Spanish Peninsula in 1808 and fought at Roliça and Vimiero. Following the retreat from Corunna the 9th buried Sir John Moore and were the last British regiment to leave Spanish soil.

Following the disastrous Walcheren expedition to the Low Countries the Regiment returned to the Peninsula and fought at Busaco, Salamanca, Vittoria, at the Siege of San Sebastián, and at Nive. The Regiment fought alongside Portuguese troops as part of Manley Power's brigade which was in Picton's Fighting Third at the Battle of Vittoria, where the 3rd Battalion took a key bridge under heavy fire.

The Regiment was sent to Canada with most of Wellington's veteran units to prevent the threatened invasion by the United States, and so arrived in Europe too late for Waterloo. The 1st Battalion participated in the Army of Occupation in France, whilst the 2nd Battalion was disbanded at the end of 1815.