Daniel MacMillan was born on 13 September 1813 on the farm of Achog, just north of Corrie on the Isle of Arran, to a crofting family.

Moving to London, he founded Macmillan Publishers, with his brother Alexander.

[3] Macmillan, with the recommendation of his brother Alexander, sent George Edward Brett to open the first American office in New York.

He married, on 4 September 1850, Frances, daughter of a Mr Orridge, a chemist in Cambridge.

They had two sons, Frederick (born 1851) and Maurice Crawford Macmillan (1853–1936).