Many secondary files show Alexander LYON & Judith HILL as the parents of the William S. LYON born in 1752.  Judith cannot be his mother because she wasn't married to Alexander LYON until 1786.  Nor can it be the case that Judith is simply our subject's second wife (and, thus, Alexander's stepmother, not his mother) because the 7 Oct 1786 Amherst Co., VA, marriage bond of Alexander LYON & Judith HILL (mentioned in a post to GenForum) says Alexander's father, William LYON of Culpeper Co., VA, "gives consent" for the marriage.  Alexander would only have needed consent to marry if he was under the age of 21 in 1786, which places his birth no earlier than 1765 (1764 if his birthday was after October 7th).  Also, if our subject died, as alleged, in Caswell Co., NC, in 1785, how could he marry in 1786? 

And what was he doing in Caswell County when his sons went to Wilkes County?  On a 1783 tax list for Culpeper Co., VA, we find a William LYON, and an Alexander LYON with son, Jacob LYON.  All three next appear together in the 1787 state census of Wilkes Co., NC.  It certainly appears that the father of William and Jacob is alive in 1787 and, like his sons, living in Wilkes County two years after his alleged death in Caswell County.  I do not see the  Alexander LYON who died in 1785 in Caswell County as our subject, nor do I see him as the husband of Judith HILL, as some show, for the again obvious reason that he couldn't have married in 1786 if he had died in 1785.

It is also the case that, so far, the descendants of the LYONs of Caswell Co., NC, are not a genetic match with the descendants of the LYONs in Wilkes Co., NC.  The Caswell LYONs are Haplogroup I2-L1498, while descendants of William S. and Jacob LYON are Haplogroup I1-P109.  People in different haplogroups cannot have had a common ancestor in thousands of years.  Not every line of LYONs who lived in these counties has had descendants Y-DNA tested, so we may yet find there are I1-P109 LYONs in Caswell County and I2-L1498 LYONs in Wilkes County, but until we do, Y-DNA test results prove the two families in the two counties do not have a common LYON patriarch.

So, the bottom line is, I think three Alexander LYONs are being mixed up here:  our subject, who (first?) appears in 1783 in Culpeper Co., VA, then in 1787 in Wilkes Co., NC; the Alexander LYON who died in 1785 in Caswell Co., NC; and the younger Alexander LYON who married Judith HILL in 1786 and lived in Amherst Co., VA.  The most common mistake made in genealogy is underestimating the number of people with the same name, and I believe that mistake has been made here.  If you think I'm wrong, please share the evidence.  I'll be delighted to clear this up.

Almost certain daughter:
.  Isabell LYON, b. 1760-65, VA
Isabell's husband, Thomas GRIMSLEY, served in the Revolutionary War alongside Jacob LYON and gave a deposition in support of Jabob's pension application (q.v.).  He is listed near Alexander LYON on the Culpeper Co., VA, 1783 tax list and next to "Elexander" LYON in Wilkes Co., NC, in the 1787 state census — all of them along with William HARRIS, also a "neighbor" of Jacob's, who also gave a deposition supporting Jacob's pension application.

1.  Jordan R. Dodd, ed.  1993.  Marriage Index: KY, NC, TN, VA, WF, 1728-1850.  Liahona Research, Orem, UT (Broderbund CD-229):  not found.

2.  Mrs. Garland C. Norris, compiler.  nd. Property Tax List of Culpeper County, Virginia, 1783.  Archives Division, Virginia State Library, Richmond, VA (online in archives of the Culpeper County, Virginia, USGenWeb site):  "List of William McClanahan, Gent."

Line	Name
71	Thomas Grimsley...
88	William Harris...
112	William Lyon...
115	Alexander Lyon, son Jacob...

The only other LYON(S) in the county is Abram LYONS, who is on a different list.

3.  Alvaretta K. Register.  2001.  State Census of North Carolina, 1784-1787.  Genealogical Publ. Co., Baltimore MD (online at Ancestry.com):

Page in Book	177			178
County	Wilkes
District	A List of the number of Souls in Capt. Johnson's Dist. July 7th, 1787. Returned by Ams. Hammon.
Page / Line in Census	1 / 13	1 / 14	1 / 16	3 / 39	3 / 40
Head-of-Household	William Harris	Jacob Lyon	William Loyon	Thomas Grimsley	Elexander Lyon
WM 21-60 yrs	1	1	1	1	1
WM under 21 or over 60	1	.	4	1	1
WF all ages	7	2	4	2	2
Blacks 12-50	.	.	.	.	.
Blacks under 12 or over 60	.	.	.	.	.

Thomas GRIMSLEY is the husband of Isabell LYON, who is almost certainly Alexander's daughter.

4.  1790 U.S. Census Index/Images (online at Ancestry.com):  not found, which suggests he is likely deceased.

5.  Kenneth Corbin.  2009.  The Leaves of the Corbin Tree.  Vol. 1 of 2.  Self-published, Louisville, KY:

p. 23	83.  Magdely Corbin, b. 8 Nov 1752, Stafford Co., VA; m. William S. Lyon, s/o Alexander Lyon and Elizabeth...  William S. LYON, b. 17 Feb 1752, Culpeper Co., VA...
The remainder of the entry is on William's page.  Corbin is probably more reliable than most secondary sources, but he has been known to err (he usually cites his sources, but seldom quotes them).

6.  Public Member Trees (online at Ancestry.com):  a few files there identify Alexander's wife as "Elizabeth OSBORNE," but without documentation.

7.  Lyon(s) Families Association of America: Database (on CD as of 22 Feb 2010; these are member submissions):  the one pedigree on the CD including William S. LYON (b. 1752) unfortunately names Judith HILL as his mother, even though it includes the 1786 marriage date.

8.  LDS.  Family Search:  Family Trees (online at familysearch.org):  the tree here gives Judith HILL as the mother of William S. LYON, even though it includes the 1786 marriage date.  I tried to correct the tree, though I think I may have botched it, but at least now I think it alerts viewers there is an issue over William's mother.