Hill Child was commissioned as a second lieutenant in the 3rd Battalion Royal Scots, a part-time Militia battalion, on 25 October 1899. The battalion was embodied for full-time service in the Second Boer War on 5 December that year, and in early March 1900 left Queenstown on the "SS Oriental" for South Africa. They landed at East London on 21 March 1900 and by July was engaged in operations against Boer Commandos in the Transvaal. Hill Child was wounded, and returned to the United Kingdom during Christmas 1900. He was promoted to lieutenant in the militia battalion on 6 March 1901, but in July was commissioned into the Regular Army as a second lieutenant in the newly raised Irish Guards. Promotion to lieutenant in the regiment came on 1 March 1902, and he was chosen to carry the colours at the first presentation of Colours to the Regiment on 30 May 1902, following which he was appointed a Member of the Royal Victorian Order (MVO). He retired from Regular service in 1909 and was placed on the Reserve of Officers in 1910.