In about 1897, he obtained a copy of the book, The Plan of the Ages, the first of the six-volume series Millennial Dawn (later called Studies in the Scriptures), written by Charles Taze Russell.

"[2] Following the convention, at a meeting at the Brooklyn headquarters, Russell announced: "The Gentile Times have ended; their kings have had their day," and added that, "At 10:30 Sunday morning Brother Macmillan will give us an address."

That year, Macmillan—along with Rutherford and other Watch Tower Society officials—was arrested, charged with violation of the Sedition Act of 1918 as a result of anti-war sentiments expressed in the book, The Finished Mystery; they were sentenced to federal prison in Atlanta, but were released and exonerated in 1919.

During the 1920s, Macmillan traveled extensively on service tours to Europe and the Middle East, for public speaking engagements and to monitor activities at branch offices around the world.

[15] By the 1930s, Macmillan, based in Brooklyn, was a "traveling representative" speaking at congregations and larger assemblies, encouraging individuals to pursue the full-time ministry.

[16] Macmillan also met with local law enforcement and government officials to explain the significance of the dozens of then-recent U.S. Supreme Court decisions which were mostly favorable to Jehovah's Witnesses.

[22] Macmillan experienced pain associated with increasing health problems, and he privately likened himself to the biblical Job, leading up to his death on August 26, 1966.

[23][24] Macmillan's funeral service was conducted by Watch Tower Society president Nathan Knorr on August 29, and he was buried at a private burial plot on Staten Island, New York.