Macmillan was the seventh of eight children of William Macmillan (1844-1911) and his wife Emily (1843-1937). He was educated at Sexey's School, Bruton (1894-7), the Quaker Sidcot School, Winscombe (1897-1901), and then at the Birkbeck Literary and Scientific Institute, London (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 he founded the Society for the Prevention and Relief of Cancer against a background of resistance. 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 by 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... [and]... panels of voluntary nurses who can be detailed off to attend to necessitous patients in their own homes." This organisation 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. 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.
Douglas 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.
- Cancer research and vivisection, 1919.
- 'SHALL WE SLAY' 1909
- 'The various animals and fish exist as part of the intricate design of nature, and in our present state of ignorance it might not be easy in every case to state the precise use of each. But whether we regard them as the direct creations of God, manifesting His manifold diversity and wisdom, or as the varied offspring of our Mother Nature, the simple fact remains that they live more or less the same as we do, having similar organs, nerves, sensations of pain or pleasure, the same joy of life and fear of death, and the same right to live their full span upon the free earth.' Douglas Macmillan, The Better Quest, 1911
- "Pastimes: Rambling - Seven routes to Stow" Birmingham Post, (Birmingham); 21 February 2004; Richard Shurey; p. 54
- Hunt, Timothy (2004). "Macmillan, Douglas (1884–1969)". Oxford Dictionary of National Biography. Oxford, England: Oxford University Press.
- Howarth, Glennys; Oliver Leaman (2001). Encyclopedia of death and dying. Taylor & Francis. p. 291. ISBN 0-415-18825-3.
- Raven, Ronald William (1990). The Theory and Practice of Oncology. Informa Health Care. ISBN 1-85070-179-2.
- "Search Blue Plaques". English Heritage. Retrieved 13 August 2008. Archived on 13th Aug 2008
- Pioneers of Their Time: The Stories of Douglas Macmillan MBE & Dame Ethel Smyth Denise Baldwin, Katherine Harding, Iris Morris, Lamorbey & Sidcup Local History Society, 1996. ISBN 0-9524661-1-2
- Shall We Slay
- Macmillan Cancer Support website
- Macmillan nurse fact sheet
- Macmillan 4x4 UK Challenge
- Douglas Macmillan Hospice