Ion Llewellyn Idriess was born at Waverley New South Wales on 20 September 1889 to parents Walter and Juliette. A miner by trade, he enlisted at Townsville on 26 October 1914 at the age of 27. He departed Sydney aboard HMAT Persic, on 21 December 1914 with the 5th Australian Light Horse Regiment.
Serving at Gallipoli, Idriess specialised in sniping. He sustained a bomb wound to his arm at Gallipoli in September 1915 and was evacuated back to Egypt. Idriess re-joined the 5th Australian Light Horse Regiment and served with his unit in Egypt and Palestine witnessing the charge at Beersheba. He was once again wounded in December 1917 in the fighting after the last battle of Gaza. Ion Idriess was consequently invalided home and returned to Australia on 15 February 1918.