William MacMillan
William MacMillan or McMillan may refer to:
- William Duncan MacMillan, American mathematician and astronomer
- William Duncan MacMillan (businessman), American businessman
- William J. P. MacMillan, Canadian physician and Prince Edward Island politician
- William McMillan (congressman), delegate to the United States Congress from the Territory Northwest of the River Ohio
- William McMillan (college president), president of Jefferson College, 1817–1822
- William McMillan (sculptor), British sculptor and medal-designer
- William McMillan (Australian politician), Irish-Australian politician, first Member for Wentworth
- William McMillan (sport shooter), American sports shooter and Olympic Champion
- William Hector McMillan, Liberal party member of the Canadian House of Commons
- William McMillan (footballer, born 1872), Scottish wing half who played for Southampton and Burnley in the late 1890s
- William McMillan (footballer, born 1876), Scottish full back/centre half who played for Lincoln City in the 1890s/1900s
- William Macmillan (minister), Moderator of the General Assembly of the Church of Scotland
- William Miller Macmillan, Scottish historian
- William L. McMillan, American physicist
- Will McMillan, American actor
- William Hutchison McMillan, Scottish mining engineer
- William Northrup McMillan, American-born Kenyan settler, adventurer, and philanthropist