Stewart was anxious to participate in military action; on 22 December 1813 his father purchased for him an appointment to a Lieutenancy in the 15th King's Hussars, which was already in action during the Peninsula Campaign. The appointment was confirmed on 6 January 1814 and Stewart joined his regiment, subsequently seeing combat during the Waterloo campaign in 1815. On 15 June 1820 Stewart was promoted to a Captain and soon thereafter retired on half pay.