Could Henry's first wife have been a daughter of this couple?
Father:  Nathan MAJORS / MAGERS (1793-1888)
Mother:  Keziah BARCUS (1795-    )
They had two daughters, one of whom was old enough to be Byron's mother:  Mary Ann MAJORS, b. 1823, OH

1a.  LDS.  Family Search: Internet Genealogy Service: IGI - International Genealogical Index (online at FamilySearch.org).

Henry THOMAS Spouse: Sarah ARMSTRONG Marriage: 21 Apr 1851, Marion, Ohio Source: Marriage Records, 1824-1964. Ohio. Probate Court (Marion County)

Nathan MAGERS / MAJORS Spouse: Keziah BARCUS Marriage: 09 Jul 1820, Knox, Ohio Source: patron submissions

1b.  Allen L. Potts.  Marion County, Ohio, Probate Court Marriage Records, Vols. I-XII, 1824-1902 (online at Heritage Pursuit):

Thomas, Henry	McHenry, Elizabeth	4/18/1844	JP Elisha Brown
Thomas, Henry	Armstrong, Sarah	4/21/1851	Samuel F. Monroe

Might Henry have had a third wife, one he married between his first wife and Sarah Armstrong?

2.  1850 Census Every-Name-Index/Images (online at Ancestry.com):  can't find Henry or Sarah (she's not with her parents).  There is a Byron THOMAS (æ 7, b. OH) living in Mount Vernon, Knox Co., OH (p. 207A), with Nathan MAJERS/MAJORS (æ 55, b. MD) and his wife, Kesiah (æ 55, b. MD) — could they be Byron's maternal grandparents?

3.  1860 Census Every-Name-Index/Images (online at Ancestry.com, Image #53-54 of 77):  Marion P.O., Village of Marion, Marion Co., OH, Roll M653_1006, pp. 425A-425B, PN 125-126, 920/919, enumerated 17 Jul 1860, official enumeration date 1 Jun 1860 (extracted by Diana Gale Matthiesen):

Henry Thomas	52	M		Merchant	2500	3000	Ohio
Sarah    "	39	F					Do
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Byron Thomas	18	M		Clerk			Ohio
Mary A. Fisher	16	F		Milliner			Do
Jennie McGuire	16	F		Do			Do
Clarinda Kempt	21	F		Do			Do

4.  1870 Census Every-Name-Index/Images (online at Ancestry.com, Image #69 of 89):  Marion P.O., Village of Marion, Marion Co., OH, Roll M693_1240, p. 103B, PN 44, 343/349, enumerated 2 Jul 1870, official enumeration date 1 Jun 1870 (extracted by Diana Gale Matthiesen):

Thomas Henry H	62	M	W	Post M Clerk			Ohio
______ Sarah	48	F	W	Keeping House	1200	159	do

5.  1880 Census Every-Name-Index/Images (online at Ancestry.com, Image #27 of 35):  Marion (Wards 2 and 3), Marion Co., OH, Roll T9_1046, p. 214C, PN 27, SD 5, ED 100, 273/280, enumerated 16 Jun 1880, official enumeration date 1 Jun 1880 (extracted by Diana Gale Matthiesen):

1880:  for an explanation of the column headings, please see What the Numbers in the Federal Census Mean (missing columns contained no data).

1

2

3

4

5

6

8

9

10

11

13

24

25

26

273

280

Thomas Henry

W

M

73

/

Millinery & Notions

OH

NY

MA

"   Sarah

W

F

58

Wife

/

"          "

MA

MA

MA

Armstrong P

W

F

84

M-in-law

/

Milliner

OH

OH

OH

Straub Mary

W

F

24

Boarder

/

"

OH

Ger

Ger

Armstrong Nora

W

F

18

Niece

/

"

OH

PA

PA

Selander M

W

F

22

Servant

/

Servant

OH

Ger

PA

The enumerator got it backwards:  Sarah was born in OH and Phoebe was born in MA.

6.  1890 Census:  the 1890 Census Population Schedules were destroyed.

7.  1900 Census Every-Name-Index/Images (online at Ancestry.com):  not found, at least not in Marion Co., OH.

8.  Teri Johnson.  Descendants of Peter Majors, Sr. (online at familytreemaker.genealogy.com/users).