I have much information on Jacob and Smallwood Smith and have researched Mark Smith. In fact, there were at least two contemporary Mark Smiths in the area so one must be careful. My conclusion is that the ancestors of Littleton Middleton Smith and Matthew Smith do not descend from a brother of Jacob and Smallwood. Genealogical statements naming Mark Smith as a sibling never have sources attached to them, although this Mark could have been a relative of some sort. We are talking about Smiths, however, and you know what that means. Smallwood Smith is the only provable brother of Jacob.
There is strong evidence that one of the Mark Smiths, the one who died first,between 1793 and 1803, was one of at least three sons of Jacob and Sarah Butler Smith, and he died apparently unmarried before his probable father Jacob died. He has several transactions associated with Jacob Smith, but this Mark Smith first appears in the records in 1784, 16 years after Jacob Smith came down to South Carolina. Mark's records are both in Newberry District and Edgefield District, as are Jacob's records and his surviving son Luke Smith's records. A superficial look at these transactions may be part of the reason prople have jumped to the conclusion there was a Mark Smith who was a brother of Jacob.
There was another Mark Smith, with wife Sally, selling land in Newberry District in early 1805. I have not traced this gentleman further. If your Mark Smith was born about 1742, he would have been older than Jacob Smith, who has been proven by land records in Loudoun County VA to have been the heir and thus oldest surviving male in that primogeniture colony. There is hardly enough time between the birth of Jacob and the birth of Smallwood for another sibling. Jacob and Smallwood's father died just after or even before Smallwood was born (Hester Smallwood Smith was pregnant at the time of her husband's death in December 1749), so there could be at most only one other surviving brother, and the posthumous child could have been a girl or Smallwood himself.
I have not found a Mark Smith on any tithe lists in Loudoun County where you find Jacob and Smallwood. The law required all males to be on the lists when they reached the age of 16. The fine for failure to sign up a required youth on the tithe list was three times the tithe itself, so compliance was probably high.
The Matthew Smiths appear to have lived in Newberry District from at least the 1780s until the 1830s when they crossed the Saluda River into the Saluda part of Edgefield District.
Benjamin West Crouch, often called the father of Saluda County, was a descendant of Littleton Middleton Smith. He wrote a note in 1962 in reply to a descendant of Jacob Smith and stated that there is no connection between the Mount Willing Smiths (Jacob named one of his plantations in Edgefield District Mount Willing) and the Matthew Smiths. Interestingly, descendents of Matthew Smith later moved into the Mount Willing area and attended Emory Methodist Church.
I suspect your Mark is one of the many other Smiths.