In 1809, the Coldstream Guards embarked for the Iberian Peninsula, and was present in all the great battles there against the Napoleonic forces, beginning with Talavera and ending with Toulouse. Having attained the rank firstly of lieutenant, then Captain, Mackinnon was appointed aide-de-camp to General Sir Edward Stopford. Whilst there he was also recorded as the constant companion of Lord Byron, then staying in Lisbon.