Where is Mercer County?

8 Mercer Counties in 8 states · 658,196 people (2025)

There are 8 counties named Mercer County in the United States, in Illinois, Kentucky, Missouri, New Jersey, North Dakota, Ohio, Pennsylvania and West Virginia. The most populous is Mercer County, New Jersey (county seat: Trenton), with 399,289 people in 2025. 6 of them are named after Hugh Mercer, Jacobite and General in the American Revolutionary War.

Map of the United States showing where all 8 Mercer Counties are, labeled by state
Every Mercer County in the United States (red), labeled with its state. Boundaries: U.S. Census Bureau, 2025. Free to use with attribution (CC BY 4.0).

All 8 Mercer Counties

Sorted by state. Each link opens the county's map.

StateCounty seatPopulation 2025Land area (sq mi)EstablishedNamed after
Mercer County, IllinoisAledo15,2295611825Hugh Mercer
Mercer County, KentuckyHarrodsburg23,7332491785Hugh Mercer
Mercer County, MissouriPrinceton3,4044541845John Francis Mercer
Mercer County, New JerseyTrenton399,2892251838Hugh Mercer
Mercer County, North DakotaStanton8,4411,0431875William Henry Harrison Mercer
Mercer County, OhioCelina42,7374621820Hugh Mercer
Mercer County, PennsylvaniaMercer107,8606731800Hugh Mercer
Mercer County, West VirginiaPrinceton57,5034191837Hugh Mercer

Questions about Mercer Counties

How many Mercer Counties are there?

There are 8 Mercer Counties in the United States, in Illinois, Kentucky, Missouri, New Jersey, North Dakota, Ohio, Pennsylvania and West Virginia.

Which Mercer County is the largest?

By population, Mercer County, New Jersey (399,289 people in 2025). By land area, Mercer County, North Dakota (1,043 sq mi).

Which Mercer County is the oldest?

Mercer County, Kentucky, established in 1785. The newest is Mercer County, North Dakota (1875).

Why are there so many Mercer Counties?

6 of the 8 are named after Hugh Mercer, Jacobite and General in the American Revolutionary War. Many counties took their names from the same founders, presidents and generals as new states were organized.

How do I find out which Mercer County an address is in?

Open the county map of the state: it has a search box for cities, towns and ZIP codes. On a phone, the "what county am I in" tool on our sister site uses your location.

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Other common county names