Mary is said to be the daughter of John & Catharine (JOHNSONBACH) ELY, but Otho & Mary are in the 1830 Census of Washington Co., PA, and John & Catharine were never known to be in Washington Co.  In 1810, John & Catherine were in Centre Co., PA, and by 1820, they were in Ohio, so I don't see how this Mary can be their daughter.

Yes, Otho & Mary do show up in Green Co., WI, in the 1850 census, as do most of John & Catharine's children, but Abraham ELY also shows up there, and he is the son of Peter & Mary Ann (HORN) ELY.  Mary, wife of Otho, is too old to be a daughter of Peter & Mary Ann, but the presence of Abraham demonstrates that it isn't just the children of John & Catharine who went to Green Co., WI, in the late 1840s.  However, if Otho's wife truly was born in Maryland, then she had nothing to do with these German ELYs from PA.  In fact, how do we know her maiden name was ELY?  I have yet to find their marriage record.  It would definitely be helpful to obtain the 1860 and 1870 census records to see whether her birthplace remains consistently MD.

UPDATE (7 Dec 2015):  I have a FamilyFinder (autosomal-DNA) match with a descendant of our subjects, as do several of my first cousins.  Given that we descend from John & Catharine, through their daughter, Elizabeth, this certainly supports the assertion that Mary is their daughter, too, or at least close kin.

1.  Family Quest Archives:  Pennsylvania Federal Census 1830:  Tioga and Washington Counties.  Heritage Quest CD M19-163 (extracted by Diana Gale Matthiesen):

1830

PA

Washington Co.

West Finly Twp.

p. 333

Othey King

210 001 - 110 010

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	4 or under	1825-1830	= John (b. 1827/8) = Son B
1 male	5-9	1820-1825	= Son A
1 male	30-39	1790-1800	= Othey
1 female	4 or under	1825-1830	= Daughter B
1 female	5-9	1820-1825	= Daughter A
1 female	20-29	1800-1810	= Mary (b. 1804/5)

Living next door to George KING.

2a.  Census Index:  U.S. Selected Counties, 1840 (Broderbund CD-316):

1840

King, John

OH

Knox Co.

Jackson Twp.

p. 404

Yes, this is the same record as the next source (the Broderbund page numbers are out of synch by one).  I'll grant that this film is a hard one to read, but it's pretty careless to have read this one as "John" when it was definitely "Otho."

2b.  Family Quest Archives:  Ohio Federal Census 1840:  Knox County.  Heritage Quest CD M704-406 (extracted by Diana Gale Matthiesen):

1840

OH

Knox Co.

Jackson Twp.

p. 403B

Otho King

311 200 100 - 001 101 000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
3 males	4 or under	1835-1840	= James (b. 1839/40) = Richard (b. 1837/8) = Henry (b. 1835/6)
1 male	5-9	1830-1835	= Jackson (b. 1832/3)
1 male	10-14	1825-1830	= John (b. 1827/8)
2 males	15-19	1820-1825	= Son B = Son A
1 male	40-49	1790-1800	= Otho
1 female	10-14	1825-1830	= Daughter B
1 female	15-19	1820-1825	= Daughter A
1 female	30-39	1800-1810	= Mary (b. ca. 1804/5)

3.  Family Quest Archives:  Wisconsin Federal Census 1850: Green and Iowa Counties.  Heritage Quest CD M432-999:  Town of Monroe Dist. No. 4, Green Co., WI, p. 224A, enumerated 13 Aug 1850 (extracted by Diana Gale Matthiesen):

147	153	Mary King	46	F		200	Maryland
		Jackson "	17	M	Farmer		Ohio
		Henry   "	14	M			Do
		Richard "	12	M			Do
		James   "	10	M			Do
		Otho    "	7	M			Do
	154	John King	22	M	Farmer		Pennsylvania
		Susannah "	15	F			Indiana

4.  LDS.  Family Search: Internet Genealogy Service: Ancestral File.