William Macmillan travelled to South Africa in 1891 with his mother and five elder siblings to join his father, who was working at the Victoria College, Stellenbosch, now the University of Stellenbosch. He attended the Boys High School, Stellenbosch, and did the first two years of the BA degree at the Victoria College, matriculating in 1901 and passing the intermediate exams in 1903. Following the death of Cecil Rhodes in 1902, Macmillan was in 1903 one of the first group of Rhodes Scholars at Oxford University in England.