He attended Hamilton public schools before moving to Detroit, Michigan, in 1855 to embark on a career in business.

He was a dominant figure in the party, leading the business interests especially in opposition to Progressive reformer Governor Hazen S. Pingree in 1897–1900.

[citation needed] McMillan died in Manchester, Massachusetts, and is interred in Elmwood Cemetery in Detroit.

He was survived by his wife Mary and their six children: William Charles, Grace Fisher, James Howard, Amy (wife of Sir John Lane Harrington, British ambassador to Ethiopia),[2][3] Philip Hamilton and Francis Wetmore.

[citation needed] Through his son James Howard McMillan, James is the great-great-grandfather of Baroness Sybille de Selys Longchamps, the mother of Princess Delphine of Belgium (Delphine Michèle Anne Marie Ghislaine de Saxe-Cobourg; born February 22, 1968),[4][5] known previously as Jonkvrouw Delphine Boël, is a Belgian artist and member of the Belgian royal family.