He was the son of John Hunter and Margaret E Hunter, of Barn Hill, Letterkenny, Co. Donegal.
In 1901 the family were living in house 11, Glebe, Castlewray, Co. Donegal. By 1911, the family are still living at that address. However, William is not present. The records show the children of the family are aged 28, 26, 23, 20, 17, 15 and 15. The 20 year old is a daughter called Mina. William Mortimer Hunter would have been aged 20 in 1911. Might he have been a twin? He may not have been present in the house on the Census day.
Hunter was serving with 4th Bn. Royal Inniskilling Fusiliers as a 2nd Lieutenant when he was killed in action on 29th June 1917 aged 26.