His parents were commissioned as Salvation Army officers and served appointments in northern England.

[4] He became a Salvation Army officer in his teenage years in Canada and was first appointed to a corps in the Ontario province.

With the rank of Colonel, was appointed as chief secretary of Canada's eastern division, where he served from 4 August 1916 to 31 October 1923, shortly after many of Canada's leading officers had been lost in the Empress of Ireland disaster.

[7] After serving as the territorial commander of the Canada and Bermuda territory, General Evangeline Booth appointed McMillan to be Chief of the Staff of The Salvation Army at the organisation's international headquarters in London, succeeding Henry Mapp.

[1] Some speculated that he may have succeeded Booth as general had he not become ill. Nearly one thousand people attended his funeral in New York City.