Many questions have been raised as to how the various names taken by this family came to pass and which is the original derivation. In a vast work on Sinclair family history published in 1894, Sinclair family historian Leonard Allison Morrison gave his theory. Whether Morrison's postulation in his work entitled "History of the Sinclair Family History in Europe and America for Eleven Hundred Years" is correct or not, it is accepted by many family historians as plausible.
It should be added to his description that most historians attribute the name "Sinkler" to a phonetic spelling of Sinclair or St. Clair when the first Scottish family members arrived in the New World. Many, if not most, of the earliest Sinclair ancestors to arrive in the New World were illiterate. As their entry on these shores was recorded by the census clerks, they would have had difficulty making out the heavy Scottish accents. "Sinkler", "Sinclere", and other forms of the name may all have been various clerks' attempts to spell the name of the new immigrants.
Morrison's "History of the Sinclair Family" is an exhaustive 453 page study. Although Morrison's work is now out of print, there is a source available from which you can obtain a complete bound and photocopied version of it. Ordering information is available in the bibliography page. (Note: The source for this publication has no connection to this Sinclair History and Genealogy Web Site.)
Orthography of the Sinclair Family Name
by Leonard Allison Morrison
The authoritative manner of writing or spelling the name is Saint Clair, or its contraction St. Clair. The name of the earliest known immigrant to America of the family was spelled Sinclare, the second Sinkler, the third Sinklaire. The name of the New Hampshire family was called and spelled Sinkler, and so was a Virginia family which appeared nearly a century after. At the present time the Sinkler form of writing it has almost entirely disappeared, although it does still prevail among representatives of one branch of the family in Virginia.
Within half a century many of the descendants of the first Sinklers, living in various portions of the country have changed their name to St. Clair or Sinclair and in some instances have written it Sinclear and Sincleare. At the present time these various forms are generally understood to be the same patronymic, and the manner of writing and pronouncing it have become largely a matter of individual taste. Indeed, in public documents, the same individual's name frequently appears in each form. In the Directory of London, England, both names are inserted under the heading "Sinclair, St. Clair."
Saint Clare, Saint Clair, or its contraction St. Clair, was the original orthography as it came from the martyr Saint Clare, the hermit on the shore of the Epte. He was the first St. Clair. In Scotland both surnames appear today, still in the course of centuries. In Caithness, where the family has long been a princely, powerful, prolific, and numerous one, the name is almost universally written and pronounced Sinclair.
While both forms of this surname are now considered as correct, as each is sanctioned by use and the custom of centuries, while each appears in Scotland and England, and in offshoots of the same family in Ireland, and other countries in Europe, in Africa, Australia, the United States, and Canada, still the argument is in favor of St. Clair as the earlier, the original derivation, and the correct orthography of the name.