Millar was born on 15 June 1925 in
Norwood, South Australia, the son of Elsie (née Klaebe) and Percival John Turbill. His father, a clerk, salesman and tram driver, died of tuberculosis in 1935. His mother subsequently married William Donald Millar and he was given his stepfather's surname. As a child, Millar's family "followed an itinerant life across Victoria and Tasmania" in the aftermath of the
Great Depression. He left school at the age of thirteen, by which time the family had settled in
Hobart. As a teenager he worked as an
assayer at the
Rosebery zinc mines and as a salesman in a Hobart department store, before joining the
Postmaster-General's Department in 1940. He worked for the department as a messenger boy, post office assistant, and junior assistant telegrapher. ==Military service==