ReesClark.com
According to...
Herewith the history of the name as told by Tartan TV, a Scottish web site that probably wants to sell you something.

The Clark family

(Clark, Clarke, Clerk)

A clark was originally a man of religion, but later this became a scholar or scribe,(from the Latin clericus). The first recorded written example of the name in Scotland was Roger clericus who is recorded as holding land in Kelso in the Scottish borders between 1174 and 1178. Thomas clericus was appointed in 1246 to determine the marches (the extent of the territory) of Wester Fedale. In 1249 James the clerk witnessed a charter by Richard to BAncori of land in Dumfriesshire. Alan clericus was witness to a charter in Aberdeen in 1281.

It has to be said that these individuals are highly unlikely to be related. In those days, their "surname" was still just a description of their occupation or notable characteristics.

However by the fourteenth century the name "le clerk" or "clark" was common throughout the Scottish lowlands, and is found in dozens of documents. For instance John Clerc owned a tenement (a high rise building!) in Edinburgh in 1400, Adam Clark or Clerk was a burgess of Dundee in 1406, and so on.

(On the subject of high rises, Scotland had the world's first skyscrapers. In medieval Edinburgh the tenements on the Royal Mile by the Castle were up to 14 stories tall - some of these still exist.)

But back to our Clarks - and some noteable examples:

Richard Clark, from Montrose, became vice-admiral of Sweden, and presented a brass chandelier to the church in his native town in 1623. The name still exists in Sweden and Finland in the form Klerck. George Rogers Clark (1752-1818, who took possession of the territory northwest of the Ohio in 1783, was another noted Scottish Clark.

And the two-stroke engine, which forms the basis of all today's lawnmower and motorcycle engines, was invented in 1879 by the Glasgwegian Sir Dugald Clerk. (A Glasgwegian is someone from Glasgow. My Clarks, however, came from the opposite (eastern) side of the island.)