In 1914 European war erupted. The Australian armed forces needed doctors and Dr Mathison enlisted immediately. He was made a field ambulance captain in the Australian Imperial Force’s 2nd Field Ambulance and sailed to Egypt on 19 October 1914.
Meanwhile, discussions were underway to establish the Walter and Eliza Hall Institute. On 23 April 1915 the decision was made to offer Dr Mathison the directorship. Sadly, he died before receiving the letter.
In Egypt, after working with the 2nd Field Ambulance, Dr Mathison was temporarily assigned to the 5th Battalion as its doctor and headed for Gallipoli on 22 April 1915.
For several days he tended to streams of wounded before transferring to Helles, where losses were also severe. On 9 May, while at his aid-post, Dr Mathison was struck in the head by a stray Turkish bullet. He died in hospital in Alexandria, Egypt, on 18 May.