Donald George Mackay was born on 29 June 1870 at Yass, New South Wales, son of Alexander Mackay, owner of Wallendbeen station, and his wife Annie.Carment, David, Mackay, Donald George (1870 - 1958)], Australian Dictionary of Biography, Volume 10, Melbourne University Press (1986), pp. 289-290 Mackay was educated at Wallendbeen Public School and at Oaklands School, Mittagong. After a brief engineering apprenticeship he worked for his father until the latter's death in 1890.Carment, pp. 289-290 During 1890-99 Mackay travelled extensively abroad, and later prospected for gold in western New South Wales.Carment, pp. 289-290
