John Shaw Mackay was in the fifth year of his medical degree at Melbourne University when he enlisted on 18 August 1914. Born in 1890, he was then 24 years old and gave as his next of kin his mother, then living at 4 Fawkner St., South Yarra. He was tall for the time, 5' 10", with dak brown hair and eyes, and by religion, Church of England. He already had military training, having completed five year as a cadet, and had one year with the Melbourne University Rifles. His brother, born 1893, Kenneth Scott Mackay, enlisted one day before him. Kenneth Mackay was wounded at Gallipoli and died at sea on 28 September, 1917 on board the hospital ship Devenha. Another brother, Ronald Grey Mackay, born 1894, also enlisted on 10 August 1915. He was to become a gunner with the AIF 5th Battalion and died  on 25 August, 1921. Both Ron and John were exposed to mustard gas while at the Front  and  contracted TB. Both men subsequently died of TB related causes - Ron in  Melbourne 1921 and John in Charters Towers in 1934. Many ex-servicemen from Melbourne in fragile health went to live in Queensland post war  because of the warmer weather.

Following his military training, John Mackay was given the rank of Staff Sergeant and, on 19 October 1914 embarked from Melbourne for Egypt on A18 Wiltshire. On arrival he was placed with the 12th Australian Field Ambulance, and was with them at Gallipoli, serving for eight months. He must have seen that he would play a more useful role  finishing his medical degree, so he asked for discharge from the AIF to return to Australia to finish his medical studies. On 23 June, 1915, he left for Australia, re-enlisting on 21 January 1916. On 26 January 1916, he was given the rank of Captain, transferred back to the Australian Medical Corps. and set sail for France and the Western Front on board the H.T. Karoola on 23 June 1916. 

On 29 July, 1916, John Mackay wasc detached from the 3rd Australian General Hospital and sent back to England, where he was taken on strength with the 7th Field Ambulance, then sent to France to work at the 2nd Australian General Hospital at Wimereux. On 30 September, he was detached temporarily for duty with the 27th Battalion in the field, but in December was in hospital with Bronchitis. 

 On 3 September, 1916, he was transferred from the A.M.C. Reinforcements to serve with the 2nd Australian General Hospital, but only six days later, was attached to the No. 7 Field Ambulance. The need for medical staff was overwhelming. In the two battles fought from late August to early Septmber, Pozieres and Mouquet Farm, three AIF Divisions suffered 23,000 casualties. At Pozieres, 6,800 died, while Mouquet Farm resulted 11,000 Australian casualties. He was undoubtedly both courageous and efficient.

On 27 September,1916, John Mackay was awarded a Military Cross. The citation read

   For conspicuous gallantry and devotion to duty when in command of a bearer division. He attended to       many casualties which occurred among the personnel of of batteries and working parties, carrying out his duties in the open with utter disregard of danger. Australian Gazette 24th September 1918

On 23 August 1918, John Mackay was wounded, with a gunshot wound and was transferred to the No 3 General Hospital, British Expeditionary Force. On 11 November, 1918, he was promoted to Major and awarded a Bar to the Military Cross.

For conspicuous gallantry and devotion to duty on 18th September at Le Verguier. He moved forward with his (stretcher) bearers closely in the rear of advancing battalions and under heavy machine and shell fire and by absolute disregard for his own personal safety was able to keep in close contact with brigade regimental medical officers. By this action, the wounded were evacuated in the quickest possible manner, all congestion at regimental psts was prevented and undoubtedly, many lives were saved.

By then the war was over, but John Mackay remained in uniform until  mid 1919, returning to Australia on H.T. Borda, disembarking on 27 June 1919. In 1924, he married Cassandra Helen Webb. They had two children, Joan and Mary. He became Medical Superintendant at the Alfred Hospital, Melbourne, but ill health from the effcts of poison gas caused the family to move to Queensland. John Shaw Mackay died on 20 November 1934 at Charters Towers, Queensland, aged 44. Cassandra Mackay died on 7 March 1969,