The McLellan's came to the USA in the mid 1700's from the Highlands of Scotland.The first known were Alexander and Malcolm in the Marion, S.C. area. W.W. Sellers wrote about them in his book, excerpt is as follows:
Alexander McLellan and his brother, Malcolm, came to this country from Scotland, in the close of the eighteenth century, and settled on the lands on which some of their descendants now live and own. Alexander was married in Scotland, to Mary McKinnon, and lived there for some time after his marriage, and several children were born to them there, who died in infancy; one a lad, named John, died after he settled here. Of the children to reach maturity, there were four sons, Daniel, Duncan, Archie K., and Colin and one daughter Flora.
There was another family of McLellans in the county. The first of them known to the writer was Rev. Archie McLellan, and a blacksmith; he lived on the south side of Catfish, on Pigeon Bay; he was a local Methodist preacher; he married a Miss Buie; had and raised a considerable family of several sons and daughters. He also had two orphan nephews, son of a deceased brother whom he raised - their names were John and Angus McLellan. [Sons and daughters mentioned by Mr. Sellers: Peter, Enos, Elizabeth and a daughter who married a Mr. Moore.]
--A History of Marion County, W.W. Sellers (1902)
I find that both families that he mentioned were and are related, so check it out please, and if you find any errors , please do not hesitate to contact me via my email address.....thanks