Macmillan Brown was the younger of two daughters of John Macmillan Brown, one of the founding professors of Canterbury University College, Christchurch, and Helen Connon, the principal of Christchurch Girls' High School and first woman graduate with honours in the British Empire.

However, when Macmillan Brown was five years old her mother died suddenly, and as a result, the girls were raised by their governess.

[2] Their father moved the family home to the hill suburb of Cashmere and, apart from her time spent overseas, Macmillan Brown lived there with him until his death.

[4] Macmillan Brown was a lifelong friend of the writer Ngaio Marsh (both grew up in the suburb of Cashmere).

[5] After her father's death in 1935, Macmillan Brown left New Zealand and went to England to marry Antonio Notariello.