Elliott entered the University of Melbourne in 1898 to study law, but left in 1900 to serve in the Imperial Bushmen in the South African War. He was awarded the Distinguished Conduct Medal, and given a British Army commission, but chose to remain with the Victorian Imperial Bushmen as an attached subaltern. He returned to Australia in 1901, but went back to South Africa to serve with the Border Scouts, who patrolled remote and inhospitable areas. In December 1901, he distinguished himself in repelling a numerically superior Boer force, and received a congratulatory telegram from General Lord Kitchener. After he returned to Australia, he completed his law degree and became a solicitor. He was commissioned as a second lieutenant in the Militia in 1904, and he was promoted to lieutenant in 1905, captain in 1909, major in 1911, and lieutenant colonel in 1913, commanding the 58th Battalion (Essendon Rifles).