The eldest of the three children of Hector Munro Mathison (1850-1895), a State School headmaster, and Mary Martha Mathison (1860-1942), née Barber, Gordon Clunes Mackay Mathison was born at Stanley, near Beechworth, Victoria on 10 August 1883.
Soon after his birth, the family moved to Elsternwick, Victoria, where both his father and younger brother Robert Mackay (born 1894) died in 1895.