The regiment then fought as part of the Duke of Wellington's army for the duration of the Peninsular War against Napoleonic France. The regiments most famous battle honour was gained in at the Battle of Talavera in 1809. The regiment also participated in the battles of Douro (1809), Albuhera (1811), Badajoz (1812), Salamanca (1812), Vittoria (1813), Pyrenees (1813), Nivelle (1813), Orthes (1814), Toulouse (1814). At the Battle of Albuera it lost its Colonel, George Henry Duckworth, son of Admiral Duckworth.
The regiment was first raised in 1741 as James Cholmondeley's Regiment of Foot in Norwich, England during the War of Austrian Succession. The regiment first saw action at the Battles of Falkirk and Culloden in 1745-1746, campaigning against the Young Pretender. In 1748, it was renumbered as the 48th Regiment of Foot. The 48th took part in the French and Indian War and they received their first battle honour in the Americas at the Battle of Louisburg, although the Regiment did not receive their due honour for this until 1882. The 48th was part of General James Wolfe's capture of Quebec in 1759.
The 48th was present at the capture Martinique and Havana in the West Indies before returning to serve in Ireland in 1763.
In 1773, the 48th was stationed in the West Indies prior to the start of the American Revolution. They were later captured by the French during the war.
Suffering from loss of men in battle, captivity and disease, the 48th was repatriated back to England in 1780. The Regiment was relocated to Northampton District and then became known as the Northamptonshire Regiment.
It too was part of the Great Siege of Gibraltar from 1779-1783, and was awarded the Castle and Key emblem.