Campbell was commissioned as an ensign in the 9th Regiment of Foot on 26 May 1808. His first experience of war was under Sir Arthur Wellesley at the Battle of Vimeiro on 21 August 1808 during the Peninsular War. His battalion remained in Portugal and served under Sir John Moore during his foray into Spain, and subsequent retreat to Corunna. His battalion was not engaged at the Battle of Corunna in January 1809, but remained in reserve. Promoted to lieutenant on 15 July 1809, he took part in the disastrous Walcheren Campaign in Autumn 1809 and contracted malaria there.