mcmillan was born in 1904 in new plymouth, the eldest child of annie gertrude pearce and david mcmillan, a dairy farmer near stratford. he received his secondary education at stratford technical high school, where he was dux. with the help of the taranaki scholarship, he could afford to study medicine at the university of otago, from where he graduated mb chb in 1929. he was a medical practitioner, first practising in kurow (1929–34) and then at 115 highgate in kaikorai, dunedin (c.1935-36, after which the building was his electorate office to 1943, then again a medical practice after he had left parliament). the building was demolished after 1950 to make way for the stuart street extension.