When the 71st Highlanders, then recently transformed into a light infantry corps, arrived out in Portugal in the summer of 1810, Cadogan joined it at Mafra and assumed command in succession to Colonel Peacocke. At its head he distinguished himself on various occasions during the subsequent campaigns, particularly at Fuentes de Oñoro, 5 May 1811, when he succeeded to the command of a brigade consisting of the 24th, 71st, and 79th regiments, at Arroyo dos Molinos 28 Oct. 1811, and at Vittoria, 21 June 1813, where he fell. On the latter occasion the 71st was ordered to storm the heights above the village of Puebla, whereon rested the French left. While advancing to the charge at the head of his men Cadogan was mortally wounded. At his request he was moved so as to be able to witness the success of the charge before he died, aged 34. Cadogan was unmarried. Wellington, who was a friend, wrote of his merit and gallantry in his Vittoria despatch.