Cotton was elected Member of Parliament for Newark in 1806. He was deployed to Portugal in April 1809 and commanded a cavalry brigade in Sir Arthur Wellesley's Army. Cotton was both courageous and also splendidly dressed in battle throughout the Peninsular War and was nicknamed the "Lion d' Or" ("Lion of Gold"). He took part in the Second Battle of Porto in May 1809 and the Battle of Talavera in July 1809 and, having succeeded to his father's baronetcy in August 1809, returned home to view his estate. He returned to Portugal in May 1810 and, having been promoted to the local rank of lieutenant general and given overall command of the cavalry, fought at the Battle of Bussaco in September 1810 and then covered the withdrawal to the Lines of Torres Vedras later that year.