Guy again saw action with the 6th during the Peninsular War, fighting at Roliça and Vimeiro, and taking part in the advance and retreat of Sir John Moore. Promoted major on 1 April 1813, Campbell and the 6th fought at Vitoria (21 June 1813), and owing to the wounding of the colonel, took command of the regiment. During the subsequent campaigns, Campbell led the 6th during the Battle of the Pyrenees, and was badly wounded while leading the regiment, on 2 August 1813, to the storming of the Heights of Echalar, a feat which won the commendation of Wellington for the regiment. Campbell was breveted lieutenant-colonel on 26 August 1813 as a result.