His father became managing director of John Boyd & Co. (manufacturers of horsehair-based products), was a Somerset County Alderman, and for 15 years edited and published the monthly Castle Cary Visitor.

[2] He was prominent in the affairs of the Society of Somerset Folk in London, founding and chairing its Arts Circle which promoted interest in folklore and dialect, drama, literature and music.

[4] He edited the Society's journal, the Somerset Year Book, from 1921 to 1932 and was Director of its London-based publishing arm, Folk Press Ltd.

[2] He published the magazine The Better Quest in 1911 "devoted to truth and humaneness" which supported animal welfare and Christian vegetarianism.

[2] Macmillan argued that meat eating was a major cause of cancer and the first chapter of the Book of Genesis supported a vegetarian way of life.

He was a staunch anti-vivisectionist and believed that good health was linked to Godliness and thus opposed conventional cancer treatments that took research from animal experimentation.

[2] In a 1912 leaflet for his Society he argued that the cause of cancer was known and that the one reasonable and reliable treatment was also known but was denied by medical and surgical orthodoxy.

[2] He stated that the cure and treatment of cancer could be found with the application of vegetarian dietetic principles rather than the surgeon's knife.

[2] Members included Frederic Cardew, Robert Bell (who became president), the Duchess of Hamilton (the first patron) and Vice-presidents Lord Charles Beresford, Roy Horniman, George William Kekewich, Sir John Kirk and Lizzy Lind af Hageby.

[2] The second Crusade pamphlet written by Macmillan was The Tea-Habit in Relation to Cancer, which argued that stimulants such as caffeine and tannic acid found in tea cause inflammation in the body.

The most significant Pamphlet of the series was entitled The Cancer Mortality Statistics of England & Wales 1851-1910, published in 1913 and became known as the "blue book".

The new emblem in 1931 was a picture of a distressed woman standing outside an occupied bedroom used to reflect the pain and despair of cancer.

[2] Macmillan married Margaret Fielding Miller in 1907,[11] and the couple afterwards lived in her parents' house at 15 Ranelagh Road, Pimlico, which provided office space for the Folk Press operation and served as the first headquarters of the Cancer Relief charity.

[7][14] 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.