McKinnon was born at Terang, Victoria to Daniel Mackinnon, a member of a well known pioneering pastoral family.
[1] He was educated at Geelong Grammar School, before going to England to study at Trinity Hall at the University of Cambridge.
[3] After graduating he returned to Australia and joined the firm Strachan, Murray, and Shannon at Geelong.
[1] Following the death of his father, he resigned from the firm to oversee the running of the Marion Downs Station in western Queensland.
[1] He served on the council of the Royal Agricultural Society of Victoria and was president of the Melbourne Club in 1938.