Substantiated facts about Mary Elliott are few and far between. Her birth has been widely reported as about 1752 in Virginia. The birth date was possibly based on her marriage date of about 1767 or 1768 and the birth of her first child, John, in 1769. Beyond this there are no documents to prove these statements but a lot of "traditional" family stories that have been repeated over and over again.
Her death has been estimated about 1787, reportedly in Kentucky. We can say her death was before 1788 because Ralph Stewart married his second wife, Mary Clay, on 25 Jun 1788 in Montgomery County, Virginia.
Other clues about Mary:
Mary Elliott had a brother Richard Elliott. Ralph Stewart married about 1767 or 1768. Mary Elliott, sister of Richard Elliott of Augusta County, Virginia. 
Richard Elliott is mentioned several times in Chalkley's Chronicles of the Scotch-Irish Settlement of VA. Two of these items show family relationships that indicate the truth of the connection between Richard and Mary Ann Ellott. First, Richard Elliott received a payment in Aug 1776 from the estate of Robert Graham. Ralph Lafferty was the administrator and had also married the widow of Robert Graham. Ralph Lafferty was the uncle of Ralph Stewart who was the husband of Mary Ann Elliott.  Second, in 1777 an appraisal of the estate of James Stewart mentions a Note for Richard Elliot. Also mentioned in the appraisal is a "private balance of Ginsang" by Ralph Stewart."  
Mary Elliott was a daughter of William Elliott and Isabella Morris.  No documentation for this statement has been found.
Early Elliott family of Augusta County, Virginia
WILLIAM ELLIOTT, SR. was born 12 Jul 1699 in Ireland, and died Jul 1795 in Rockbridge County, VA. He married MARGARET. She was born Unknown in Ireland, and died Unknown in Rockbridge County, VA.  
The Will of William Elliott, Rockbridge County, Virginia Will Book 1, p. 494. This will was submitted to the Court of Rockbridge County on July 7, 1795.
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