While in Grahamstown, Macmillan did pioneering work on poverty among white South Africans. He was the only English-speaking delegate at the Dutch Reformed Church congress on White Poverty, which was held at Cradock, Eastern Cape in 1916. Macmillan's first published work appeared anonymously as a pamphlet, "Sanitary Reform for Grahamstown" in 1915. This was followed in the same year by "Economic Conditions in a Non-industrial South African Town". In 1918, Macmillan did substantial fieldwork on white poverty in rural areas, resulting in lectures and a book, "The South African Agrarian Problem and its Historical Development", published in Johannesburg in 1919.