John Michael Mackay was the son of Joseph Mackay and his wife Charlotte, nee Piper. The couple married in 1890 and had nine children: Joseph (1890); Theresa (1892); William (1894); Ada (1897); Winifred (1900); Thomas (1904); Anthony (1908); Mary (1909). John. known as Jack, was born on 28 December, 1895, making him the fourth in his family. He was born at Mount Morgan, Queensland, but the family seemed to move around frequently. In 1916, when he enlisted , Jack was living in Western Australia , giving his address as the Post Office at Merindel.

He enlisted on 23 February, 1916, and was sent for training as a sapper with the No. 6 Tunnelling Corps Reinforcements. He may well have had some mining experience. He had certainly been at Kalgoorlie, where he had trained in the pre-war years with the 84th Infantry. He was just over 21 years old and single. Following training, he was attached to the 6-10 Reinforcements of the 5th Pioneer Battalion.

The Pioneer Battalions were the 'handymen' of the AIF. They were involved in labouring and construction tasks in support of the troops, with one Pioneer Battalion attached to each Division of the military. They were engaged in the construction of roads and light rail, building duckboard paths, digging communictaion trenches, often undertakgin the work in the direct line of fire. The 1916 intake began with veterans of the Gallipoli campaign, but the majority were tradesmen, miners and craftsmen from South and Western Australia. 

Following training the 5th Pioneer Battalion Reinforcements left from Fremantle on HMAT Port Melbourne A 16, embarking on 30 October, 1916, and disembarking in England on 28 December. The voyage over was difficult for Jack Mackey. On 8 December, he was sent to the ship's hospital and released the next day; on 11 December, he was again admitted and discharged the following day; on 18 December, he was again ill and again admitted to hospital, but it is impossible to tell from his service record whether he was just prone to sea sickness or whether there was some underlying health condition at this stage. 

After a further two months' training in England, the battalion was shipped to France on board the SS Golden Eagle, leaving Folkestone on 28 February and landing the next day. Again, Jack Mackay was sick on board ship and was admitted to hospital. He did not rejoin his battalion until 7 March, 1917, where they were serving at the Western Front. Only ten days later, however, he was ill, this time with mumps, and was sent to hospital in France, before returning to the front on 30 April. He was returned to England on 29 July, 1917, and was admitted to the 5th Southern General Hospital at Plymouth. He had been diagnosed as having Lymphatic Leukaemia and was not fit for further service. On 17 September, he was in 3rd Auxiliary Hospital, this time with enlargement of the spleen and from here, he was discharged and sent back to Australia.

He rteurned to Western Australia aboard the HT Port Lyttleton and was discharged from the army on 4 January, 1918.  He applied for a pension and was given 30/- per fortnight, payable from 5 January, 1918. In 1924, he was again working as a grocer at Meriden, W.A., but in 1924, had shifted to Victoria, where he was living at St. Kilda West and was a clerk. He never married.  In 1943, he was still in St. Kilda, but in 1949, he was living at 48 George St., East Melbourne and working as a manager. He died at Melbourne on 23 May, 1960, aged 64, and is buried the Melbourne General Cemetery, Carlton.