Pakenham, as adjutant-general, joined his well known in-law, the Duke of Wellington, in the Peninsular War. He commanded a regiment in the Battle of Bussaco in 1810 and in 1811 fought in the Battle of Fuentes de Onoro to defend the besieged fortress of Almeida, helping to secure a British victory. In 1812 he was praised for his performance at Salamanca in which he commanded the Third Division and hammered onto the flank of the extended French line. He also received the Army Gold Cross and clasps for the battles of Martinique, Busaco, Fuentes de Oñoro, Salamanca, Pyrenees, Nivelle, Nive, Orthez, and Toulouse.