He was educated at the Royal Military Academy, Woolwich and commissioned into the Royal Artillery in 1875. He served in the Second Anglo-Afghan War from 1878 to 1880, and in the Anglo-Egyptian War of 1884. He was promoted to Major in 1892. Thomas was promoted to Colonel in 1899 and fought in the Second Anglo-Boer War between 1899 and 1901, during which he was twice mentioned in dispatches. In 1901 he was made a Companion of the Distinguished Service Order, and a Companion of the Order of the Bath in 1904. Between 1909 and 1911 he was brigadier-general of the 3rd Division Artillery, before serving as the brigadier-general of 24th Division Artillery between 1914 and 1915.