256 documented Spellings of MacMillan.
Regardless of nationality, people were not particular about the spellings of their surnames until the 20th century. Nor were the records of the time. Therefore, there is a great variety of spellings of any name. Surnames weren't employed in Western Europe until the 12th century and then only among the nobility. A man would have been known by his given name and his father's name, a nickname or a reference to where he lived or was born. Highlanders might not have adopted a surname until the late 18th or early 19th centuries, sometimes only when they emigrated.
Another significant factor in spelling inconsistency was emigrants from the British colonies would often be illiterate. If Scottish Highlanders, they'd often only have spoken Gaelic or "Erse" as it was then referred to. In either case, the spelling of their names would have been interpreted phonetically by English-speaking clerks. This also applied to Scotland itself, where Gaelic names pronounced in the wide variety of local Scottish accents received an equally wide variety of spellings.
The "official" spelling of MacMillan (as applied to the Chief and clan) wasn't consistently applied until the mid-19th century.
MacMhaolain:
MacKmallen
Mackmellon
Mackmillen (1623)
Mackmillion
MacKmolene
Mackmulane (1540)
Mackmyllan
MacMallen
Macmilan
Macmilian
MacMillan
Macmillan
Macmillen
Macmillian
MacMillion
Macmillion (1804)
Macmillon (1661)
MacMolan (1263)
Macmolane (1452)
MacMulane
MacMulen (c.1470)
Macmullan
MacMullen
Macmullen
MacMullin
Macmullin
Macmwlane
MacMulling
MacMylan (1745)
MacMyllan (1745)
Macquemullan
Makmillan (1644)
Makmilland (1693)
Makmillane (1612)
Makmolan (1505)
Makmollane
Makmulane (1506)
Makmullane (1526)
Makmyllan
MakMyllane (1488)
Makmyllane (1488)
Makmyllen (1555)
Maximilian (1670)
Maxmilan (1654)
Mckmillan
McKmillan (1745)
McMallan
McMallen
McMallin
McMalon (1860)
McMelane
Mcmelane (1539)
McMelayn (1606)
McMelayne (1606)
McMelen (1533)
McMellan
McMellen
McMellian
McMellion
McMellon
Mcmellon (1870)
McMelon (1850)
McMeylane (1511)
Mcmeylane
McMilan
McMilane (1526)
McMilean (1791)
McMilen
Mcmilen (1826)
McMileon
McMilion (1828)
McMillain
McMillaine
McMillan
Mcmillan
McMilland (1745)
McMillane (1632)
Mcmillane (1653)
McMillean
McMilleen
McMillen
Mcmillen (1641)
McMillian (1843)
McMilliaon
McMillin
McMilling
McMillioan
McMillion (1824)
Mcmillion
McMilliond (1824)
McMillon
McMillond (1824)
McMillone
McMillun
McMilyon (1781)
McMolan (1505)
McMolane (1505)
McMolen (1589)
McMollan
McMolland (1606)
McMollane (1611)
McMollem
McMollen
McMollim
McMollin
McMollone
McMolyne (1542)
McMouln
McMoyland (1505)
McMoylane (1615)
McMoylen (1620)
Mcmoyllen (c.1636)
McMuillane
McMuillen
McMuilline
McMulan
McMulane (1506)
McMuleon
McMuline
McMulion
McMulione
McMullain
McMullan (1745)
McMulland
McMullane (1539)
McMullen
McMulleon
McMullian
McMullin
McMulling
McMullins
McMullion
McMullione
McMullon
McMullwne
McMulyione
McMwlane (1473)
Mcmylen (1576)
McMyllane (1488)
McVellen (1588)
Mickmillan (1489)
Muckmelon (1850)
Total: 143
MacGhillemhaoil
MacElmail
MacElmeel
MacElmoyle
Macgilbile
MacGilmole
Macgilveil
MacIgheil
MacIgheill
MacIlemoyle
MacIllemhaoel
MacIllemhool
MacIlleveole
MacIlmoil
Macinvail (1750)
Mackenbale (1771)
Macklvaill
MacLeval (1774)
Magivul (1760s)
Makgillemuil
McAlveal (1780)
McClevael (1771)
McClevail (1772)
McElbale (1780)
McElmeel
McGillemoil
Mcgillemoill
McGillemoill
McGillemoyll
McGillemule (1498)
McGillevoyll
McGillimoyle (1602)
McGilmole (1614)
McGilmoyle
McGilmwell
McGilvaoil
McGilveall
McGilveill
McGilvoyl
McIgheil
McIgheill
McIlevail
McIleveule
McIlevoil
Mcile voill
McIlimhewil
McIlivail
McIliveule
McIlivile (1777)
McIliviul
McIivoil (1733)
McIlivoile
McIllemoyll (1636)
McIllemull
McIllevail
McIllevoill
McIlliveaill
McIllvaoil
McIllvoll
McIllywoul
McIlmail
McIlmale
McIlmoil
McIlmoill
McIlmoyle
McIlovyll
McIluvail
McIlvail (1778)
McIlvaile (1764)
McIlvaille (1761)
McIlvailly (1769)
McIlvale (1780)
McIlvall (1762)
McIlveall
McIveil
McIlveile
Mcilveoill
McIlveoll
McIlvoail
McIlvoi
McIlvoile
McIlvoill
McIlvoille
McIlvole
McIlvoll
McIlvoyel
McIlvoyle
McIlvoyll
McIlvyle
McIlwoyll
McImale (1786)
McInvale (1769)
McInvail (1784/87)
McInvaille (1772)
McIyeall
McIyeile
McKanvail (1760)
McKenvail (1761)
McKilvoile
McLivole
McQuevail (1762)
McUlvoyl
McUlvoyll
MhicIghaill
MhicIgoull
Total: 104
Others
McGeyll (1686)
McIllmilleine (1691)
McIlmalein (1670)
McIlmalene (1609)
Mcilmiling (1653)
Mcilmolyne (c.1636)
McKevoil
Macmoil
Mac-na-maoile
Total: 9