Mercer County, Ohio

Mercer County is a county in Ohio, United States, about 58 miles northwest of Dayton on the Indiana border. It had a population of 42,528 at the 2020 census, an estimated 42,484 by 2024, and covers 473 square miles (1,225 km2) — 462 square miles of land and 11 of water — centred near 40°32′N 84°38′W.

Celina is the county seat and the county’s only city; its postal code is 45822. The county holds 1 city, 8 incorporated villages, 1 census-designated place and about a dozen smaller unincorporated communities across 14 townships, and it runs on Eastern time. Its FIPS code is 39107.

Towns & Villages in Mercer County

Mercer County has 23 named places: one city, eight incorporated villages, one census-designated place (CDP) and thirteen unincorporated communities with no local government of their own. The table below gives each place’s type, home township, 2020 census population where one exists, and ZIP code.

Fort Recovery straddles Gibson and Recovery townships, so its address and its post office both sit inside a place that spans two township lines. Burkettsville sits mostly across the county line in neighboring Darke County; only its eastern edge, and St. Bernard’s Catholic Church on the Mercer County side of Main Street, fall inside Granville Township. Chickasaw shares its 45826 ZIP with the rural mail routes on Celina’s south side rather than having a wholly separate delivery area.

Townships

Mercer County’s 14 civil townships range from Jefferson, which contains most of Celina, down to Black Creek, which has no incorporated place at all.

ZIP Codes

• 45822 — Celina (also serves Chickasaw’s rural routes and southern Jefferson Township)
• 45828 — Coldwater
• 45883 — St. Henry
• 45846 — Fort Recovery
• 45882 — Rockford
• 45862 — Mendon
• 45866 — Montezuma
• 45860 — Maria Stein
• 45310 — Burkettsville (shared with its larger share in Darke County)

Carthagena, Cassella, Chattanooga, Cranberry Prairie, Erastus, Macedon, Mercer, Neptune, Padua, Philothea, St. Rose, Sebastian and Tama have no ZIP code of their own; none has ever had its own post office, so mail runs through the nearest incorporated village.

Geography

The county sits on flat, glaciated farmland at the edge of the historic Great Black Swamp, with elevation ranging from 1,071 feet on the southern border to 780 feet where the St. Marys River crosses into Auglaize County. Its dominant feature is Grand Lake St. Marys, a 13,500-acre reservoir built between 1837 and 1845 to feed the Miami and Erie Canal — the world’s largest artificial lake at the time it was dug, straddling the Mercer–Auglaize line in northeastern Jefferson Township. The St. Marys River drains north out of the lake, and the Wabash River rises from small headwater streams in western Mercer County before crossing into Indiana.

Mercer County’s neighbors are Van Wert County to the north, Auglaize County to the east, Shelby County to the southeast, Darke County to the south, Jay County, Indiana, to the west, and Adams County, Indiana, to the northwest: Van Wert County, Auglaize County, Shelby County, Darke County, Jay County and Adams County.

Demographics

Mercer County had a population of 42,528 at the 2020 census, up slightly to an estimated 42,484 by 2024 — essentially flat over the decade, unlike most of rural Ohio. Celina (10,935) and Coldwater (4,774) are the two largest communities, together holding more than a third of the county’s residents. Median household income was $76,782 in 2024, well above the statewide figure, with a poverty rate of 7.29% against a national average near 12.5%. Land density works out to roughly 90 people per square mile, concentrated around Celina and the Coldwater–St. Henry corridor to its south.

Economy & Local Life

Manufacturing is the county’s largest employer by far, with 6,213 workers and the highest average pay of any sector at $58,683 a year; Celina Aluminum Precision Technology, Fort Recovery Industries and J&M Manufacturing are the biggest named employers. Health care and social assistance employs 2,709 people, anchored by the Mercer Health hospital system in Coldwater, and retail trade employs 2,219. WSU’s Lake Campus, a branch of the Dayton-area public university, sits on the north shore of Grand Lake St. Marys between Celina and St. Marys and has offered two- and four-year degrees since 1962. Mercer County’s heavy German Catholic settlement in the mid-to-late 1800s left it inside the region known locally as the Land of the Cross-Tipped Churches, for the tall steeples visible for miles across the farmland. The Lake Festival, held the last weekend of July in Celina, centers on Grand Lake St. Marys and includes one of the world’s largest gatherings of amphibious Amphicars; the Mercer County Fair runs the second week of August in Celina.

History

Mercer County was formed on April 1, 1820, from Darke and Shelby counties and organized as a functioning county government on January 2, 1824. It is named for General Hugh Mercer, a Revolutionary War officer killed at the Battle of Princeton. Its earliest recorded history is military: on November 4, 1791, a Native American confederacy destroyed Arthur St. Clair’s expedition near what is now Fort Recovery, the U.S. Army’s worst defeat by Native American forces. General Anthony Wayne built a fort on the same ground in December 1793, and the confederacy besieged it again on June 30, 1794, without retaking it. The Treaty of Greenville, signed after Wayne’s victory at Fallen Timbers in 1795, used Fort Recovery as a fixed point on the boundary between Native American and United States territory. Land south of that treaty line stayed part of Darke County until 1848. German Catholic immigrants settled the county’s southern townships from the 1830s onward, building the parish network that still shapes local life. Grand Lake St. Marys, dug by hand and mule between 1837 and 1845, briefly held oil derricks in the lake itself starting around 1891, in one of the first offshore oil operations in North America.

Getting There

No interstate highway crosses Mercer County. US Route 127 runs north–south through the county and serves as Celina’s Main Street; US Route 33 crosses the northeast corner through Rockford and Neptune. State routes 29, 118, 119, 197, 219 and 703 fill in the network between the townships.

• Lima, Ohio — about 30 miles east
• Dayton, Ohio — about 58 miles southeast
• Fort Wayne, Indiana — about 45 miles northwest
• Van Wert, Ohio — about 20 miles north

The nearest commercial airport is Fort Wayne International Airport in Indiana, about 45 miles northwest; Dayton International Airport is roughly 60 miles southeast. The closest Amtrak service is the Capitol Limited’s stop in Waterloo, Indiana, around 65 miles to the north.

Frequently Asked Questions

Where is Mercer County, Ohio?
It sits in west-central Ohio on the Indiana border, about 58 miles northwest of Dayton and 30 miles west of Lima.

What is the population of Mercer County?
Mercer County had a population of 42,528 at the 2020 census, estimated at 42,484 in 2024.

How big is Mercer County?
The county covers 473 square miles (1,225 km2), of which 462 square miles are land and 11 are water.

What is the county seat of Mercer County?
Celina is the county seat and the county’s only city.

How many towns and villages are in Mercer County?
Mercer County has one city, eight incorporated villages, one census-designated place and thirteen unincorporated communities, spread across 14 townships.

What ZIP codes are in Mercer County?
Nine ZIP codes cover the county’s incorporated places, led by 45822 for Celina; several small unincorporated hamlets have no ZIP of their own and use a neighboring village’s code.

What is Grand Lake St. Marys?
It is a 13,500-acre reservoir built in the 1830s and 1840s to feed the Miami and Erie Canal, once the largest artificial lake in the world; it now sits at the center of Celina’s recreation and tourism economy.

More counties in this state: Towns and villages in Ohio.