He was born on 10 August 1884, in Castle Cary, Somerset, England, the seventh of eight children of William Macmillan and his wife Emily. He was educated at Sexey's School, Bruton, the Quaker Sidcot School, Winscombe, and then at the Birkbeck, University of London in 1901. Macmillan entered the civil service in London in 1902 and worked in the Board of Agriculture and, later, the Ministry of Agriculture and Fisheries, retiring as a staff officer in 1945. The death of his father from cancer, in 1911, left a deep impression on Macmillan, and the following year, despite having no medical background himself, he founded the Society for the Prevention and Relief of Cancer, with a donation of £10. The National Health Service had yet to be established, and registration of nurses was not introduced until 1919. At the time no health and safety acts had been passed by Parliament and public health had yet to become a priority for the state. In setting up the society, Macmillan "wanted to see homes for cancer patients throughout the land, where attention will be provided freely or at low cost, as circumstances dictate... ... panels of voluntary nurses who can be detailed off to attend to necessitous patients in their own homes." Macmillan managed the charity, along with other volunteers, while working full-time as a civil servant. In 1924 he moved to Knoll Road, Sidcup, where he would live until 1966. In 1930 the charity took on its first full-time member of staff. The organisation he founded has since flourished and is today known as Macmillan Cancer Support. A blue plaque was erected to honour him at his former residence of 15 Ranelagh Road, Pimlico in 1997 and another in 2019 at his birthplace in Castle Cary. In October 2010, The Bexley Civic Society invited the Mayor of Bexley, Cllr Val Clark, to unveil another plaque on his house in Knoll Road, Sidcup where he lived for 30 years. Macmillan was a vegetarian. In 1909, he wrote an open letter to all Christians entitled Shall we slay? which encouraged orthodox Christians to consider vegetarianism. Macmillan died of cancer on 9 January 1969 at his home Carylande, Ansford in Castle Cary, aged 85.
