Matheson joined the Hertfordshire Militia before being commissioned as a second lieutenant into the Bedfordshire Regiment on 15 December 1888. On 2 June 1894, Matheson transferred from the Bedfordshires, in which he was then a lieutenant, to the Coldstream Guards as a second lieutenant again, and was promoted lieutenant in that regiment on 1 December 1897. He served in the Second Boer War, as adjutant of the 1st battalion from 1899 until May 1902. The battalion took part in the battles of Belmont (23 November 1899), Enslin, Modder River (28 November 1899) and Magersfontein (11 December 1899), and he was mentioned in despatches, and promoted to captain on 20 April 1901. Following the end of the war in June 1902, Matheson left Cape Town in the SS "Carisbrook Castle" in September 1902, arriving at Southampton early the following month. At the outbreak of the First World War, Matheson went with his regiment to France and fought in several actions, being promoted to Lieutenant Colonel in 1915 in command of the 3rd Battalion.