Millanson / Melanson / Melancon
Mac-Millan means "Son of Millan", so Millan-son is just an English form of the same name, and as with all names in pre-19th century records it appears with a variety of spellings. Its earliest appearance so far found is in the years 1505 to 1521 when Andree Mulansone / Andree Melanesone was a crown tenant in Litill Tary in Morayshire. This is an area to the east of Inverness and not really therefore in the Gaelic-speaking Highlands; which probably explains why this English form of the name appears there. It's notable however that this man holds the tenancy of his farm jointly with a McGillemoyll - a phonetic form of the Gaelic MacGhillemhaoil - which strongly suggests that he was indeed a MacMillan. In 1580 Gilbert Millansoun was a royal tenant in Moy of Calder/Cawdor in Nairnshire - the county between Moray and Inverness - and he is probably the same man as, or the father of, Gilb. Millanesone, one of the occupiers of Moy in "vic. Elgin et Forres" (i.e. the sheriffdom of Moray) in 1620.
The name Mellanson appears in Nova Scotia in the late 17th century and traditions there say that it was of Scottish origin. The colony was under French control however for much of the first half of the 18th century, and some descendants of the brothers Pierre and Charles Melanson are thought to have settled in France after the British finally expelled the French from Nova Scotia, where their surname became Melancon. Nobody has yet been able to confirm this family's supposed Scottish connection - which if true would almost certainly make them MacMillans in origin - and an excellent summary of what is currently known about them can be found in the excerpt from Marg Melanson's "The Melanson Story" - a PDF file. You can find out more about "The Melanson Story" online.
The names Millanson / Melanson etc. are very rare now in Scotland itself, so most members of this sept are to be found abroad - where they'll find a warm welcome in any regional or national Clan MacMillan branch or society.