When was Mercer County founded?

Mercer County was founded in 1820, formed from Darke County and Shelby County, and named after Hugh Mercer.

What is the county seat of Mercer County?

The county seat of Mercer County is Celina.

Is there an online parcel map for Mercer County?

Yes. A public source publishes a parcel map of 25,762 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.

Has Mercer County's boundary or name changed?

Mercer County was created in 1820 from Darke County and Shelby County. Since then the Atlas of Historical County Boundaries records 6 later changes, chiefly involving Allen County, Van Wert County and Darke County.

Is my lot in a flood zone?

We do not have flood maps for Mercer County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

Mercer County, Ohio: parcels, history and offices

Where it is

Mercer on the map of Ohio

Ohio has 88 counties and county equivalents, each with its own records office. Shapes with a page are links.

Counties of Ohio · Mercer highlighted

History

Formed in 1820 from Darke County and Shelby County

An old deed may name a county that no longer exists, or a boundary that has since moved.

Mercer County was created in 1820 from Darke and Shelby Counties, named after Hugh Mercer, though it was initially attached to Darke for administration. It was fully organized in 1824 when detached from Darke, and over the following decades exchanged land with neighboring counties Darke, Shelby, Allen, Van Wert, and Auglaize as boundaries were adjusted.

Boundary changes from the Newberry Library, Atlas of Historical County Boundaries.

1820
Mercer County created Mercer created from Non-County Area 6 attached to Darke and Non-County Area 7 attached to Shelby; Mercer not fully organized, attached to Darke for administrative and judicial purposes.
from Darke from Shelby
1 April 1820 · Ohio Laws 1819, 18th assy., loc., ch. 37, secs. 1-2/pp. 90-91; Chase, 3:ch. 429/pp. 2134-2135
1824
Changed Mercer fully organized, detached from Darke. Van Wert detached from Darke, attached to Mercer "for judicial purposes."
Darke Van Wert
2 January 1824 · Ohio Laws 1823, 22d assy., loc., ch. 35/p. 49; Chase, 3:ch. 443/p. 2138
1826
Gained territory Mercer gained from Shelby.
from Shelby
1 March 1826 · Ohio Laws 1825, 24th assy., loc., p. 62; Chase, 3:ch. 453/p. 2141
1828
Changed Allen detached from Shelby, attached to Mercer "for judicial and other purposes."
Allen Shelby
1 March 1828 · Ohio Laws 1827, 26th assy., gen., p. 3; Chase, 3:ch. 460/pp. 2143-2144
1831
Changed Allen fully organized, detached from Mercer.
Allen
1 March 1831 · Ohio Laws 1830, 29th assy., loc., p. 94; Chase, 3:ch. 470/p. 2145
1837
Changed Van Wert fully organized, detached from Mercer.
Van Wert
18 March 1837 · Ohio Laws 1836, 35th assy., loc., p. 273
1848
Exchanged territory Mercer exchanged with Darke, lost to creation of Auglaize, and lost to Allen and Van Wert.
with Darke with Auglaize with Allen with Van Wert
1 March 1848 · Ohio Laws 1847, 46th assy., loc., secs. 1-4, 7/pp. 128-129

Land and parcels

What the maps cover in Mercer

Who publishes each layer, how good it is, and when we last checked.

Parcels Good enough

Lot lines and tax-map numbers for 25,762 parcels. Drawn for tax purposes: usually within a few feet of a survey, but not a survey.
edited 14 October 2026 · checked 15 October 2026
Address points Official

One point per address, 21,596 in all from Ohio Geographically Referenced Information Program (OGRIP), Ohio DAS/OIT (state GIS office).
edited 24 June 2025 · checked 13 October 2026
Buildings Reference

Roof outlines traced from aerial imagery from FEMA Response Geospatial Office with Oak Ridge National Laboratory (ORNL). Good for seeing what is on a lot; not accurate enough to set a fence from.
checked 18 October 2026

Official: from the office that keeps the record
Good enough: a reliable statewide or national copy
Fallback: a guide only; check the official record
Missing: nothing public found yet

People and land

Mercer by the Census

Population

42,528 2020 Census count

Housing units

17,888 2020 Census count

Land area

462 square miles

Water area

11 square miles

People per sq mi

92 population ÷ land area

Source: US Census Bureau, 2020 Census; US Census Bureau, 2020 Gazetteer Files. Not endorsed or certified by the Census Bureau.

Rules and offices

Mercer County's official site

We have found the county's site but not its offices yet; start there.

County government

County government
The official website of Mercer County.
mercercountyohio.org

See your own lot on these maps

Open Mercer County on Parcel Chart's free map and click your parcel. Save it with a free account; the full record of your place (site map, projects, documents) is Pro, in early access.

Open the free map no sign-in Pro early access

Questions

Common questions about Mercer County

When was Mercer County founded?: Mercer County was founded in 1820, formed from Darke County and Shelby County, and named after Hugh Mercer.
What is the county seat of Mercer County?: The county seat of Mercer County is Celina.
Is there an online parcel map for Mercer County?: Yes. A public source publishes a parcel map of 25,762 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.
Has Mercer County's boundary or name changed?: Mercer County was created in 1820 from Darke County and Shelby County. Since then the Atlas of Historical County Boundaries records 6 later changes, chiefly involving Allen County, Van Wert County and Darke County.
Is my lot in a flood zone?: We do not have flood maps for Mercer County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

Nearby

Mercer CountyOhio