Cowell joined his father’s regiment, the Coldstream Guards, and fought in the Peninsular War at Fuentes de Oñoro, Ciudad Rodrigo, Salamanca and Vittoria. Cowell subsequently wrote a memoir of his experiences in the war . He fought at the Battle of Quatre Bras in 1815 but an attack of dysentery led him to miss the Battle of Waterloo. Peacetime postings included France, Manchester, Gibraltar and Malta. He was promoted to lieutenant-colonel in 1830.