Mercer County, Ohio

Founding: 1824 (declared in 1820)
Parent county: Unorganized territory

Namesake: Hugh Mercer, Continental Army general
Seat: St. Marys (1824–1839); Celina (1839–)

Land area: 462 square miles

Population (2020): 42,528

Population (historic): 1,110 (1830); 14,104 (1860); 21,808 (1880); 28,021 (1900); 25,096 (1930)

Periods of population growth: 1820–1840; 1850–1900; 1930–2000; 2010s

Subdivisions: One city; seven villages; 14 townships

National Register listings: 30

Pre-1940 residences (estimated): 3,578 (20.3%)
Pre-1940 housing survival rate: 56.8%
Pre-1860 housing survival rate (1940): 3.6%

Farm housing in disrepair (1950): 3.4%

Nonfarm housing in disrepair (1950): 3.5%
Average farmhouse value (1930): $1,393
Average outbuilding value (1930): $1,642

Number of farms (1920): 3,040
Average farm size (1920): 77.8 acres
Sources of settlement: Germany and Pennsylvania
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Celina

Namesake: Salina, New York

Founding: 1834 (platting); 1840 (post office)

Population (2020): 10,935

Population (historic): 222 (1850); 1,346 (1880); 2,815 (1900); 4,664 (1930)

Periods of population growth: 1834–2020
Pre-1940 residences (estimated): 657 (13.3%)
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Outstanding Buildings

1. *Martin Winkler Farm (c. 1885). 12561 Wabash Road. Black Creek. Italianate.
2. *George Dellinger Log House (1852). 799 Winkler Road. Black Creek.
3. *Joseph Speck Farm (1872). 3459 State Route 119. Granville. Razed.
4. *Henry Aufderhaar Log House (c. 1870). 3066 Carmel Church Road. Jefferson. Razed.
5. *Maria Stein Convent (1892/1910). 2291 St. John's Road. Marion. Gothic Revival.
6. *Bernard Knapke Farm (1874). 2570 St. John's Road. Marion.
7. *Joseph Ungruhn Farm (1882). 6926 State Route 119. Marion.
8. *St. Aloysius Catholic Church (1875–1878). 6032 State Route 274. Marion. Romanesque.
9. *Henry Grieshop Farm (c. 1870). 8739 State Route 274. Marion.
10. *St. Charles Seminary (1919–1925). 2860 US Highway 127. Marion. Gothic Revival.
11. *St. Peter Catholic Church (1904–1906). 3512 St. Peter Road. Recovery. Gothic Revival.
12. *Philo Wallischeck Farm (1867). 80 Zehringer Road. Recovery. Razed.
13. Isham Robbins Farm (1884). 11020 Gallman Road. Union. Italianate.