In 1808 Sir Arthur Wellesley appointed Campbell his senior aide-de-camp, when he took command of the expeditionary force destined for Portugal, and sent him home with the despatches announcing the victory at Roliça on 17 August Campbell, however, windbound and hearing the guns, disembarked, and was present at the Battle of Vimeiro. Sir Harry Burrard then gave him the Vimeiro despatch, and Campbell was promoted a major in the army by brevet on 2 September 1808, and major of the 70th Foot on 15 December 1808. On the same day he was appointed an assistant adjutant-general to a division of the reinforcements intended for the Peninsula. He was present at the passage of the Douro, at the battles of Talavera and Busaco, and was promoted lieutenant-colonel by brevet on 3 May 1810. He was frequently engaged during the pursuit of Marshal Masséna and was present at the Battle of Fuentes de Oñoro and at Salamanca. He obtained the post of assistant quartermaster-general at the headquarters of the army in the Peninsular, at Wellington's special request, in the spring of 1812, and acted in that capacity until the end of the Peninsular War, doing much, it is said, to smooth Wellington's relations with the quartermaster-general, George Murray. He was present at the storming of Badajoz and in ten general actions, for which he received the Peninsular Gold Cross and six clasps.