Educated at the Belfast Royal Academy, Girdwood was commissioned into the Cameronians (Scottish Rifles) as a second lieutenant on 20 May 1899. He served in the Second Boer War in South Africa with the 2nd battalion of his regiment. They took part in the Ladysmith relief force, including the battles of Colenso (December 1899), Vaal Krantz (February 1900) and the Tugela Heights (February 1900). During this advance, he was promoted to lieutenant on 25 January 1900. He served in the Natal from March to June 1900. Following the end of the war, he left South Africa for England in July 1902.