Mercer County, Ohio: History
Mercer County was created in 1820 from land that had been attached to Darke and Shelby counties. Land south of the Greenville Treaty Line was still part of Darke County. An act establishing and fully organizing Mercer County took place on January 2, 1824. In 1837 Van Wert County was detached and the county line established is the current northern border of Mercer County. In 1839 Celina was established as the seat of Mercer County; St. Marys was the previous seat. In 1848 the area south of the Greenville Treaty Line to the current southern county line, was attached. When Auglaize County was formed, Mercer County's eastern border was moved 6 miles (9.7 km) west with the exception of the area south of the Greenville Treaty Line. This created the sharp point at Mercer County's southeast corner and was the last county line modification.
In the mid to late 1800s, Mercer County became home to many German immigrant farming families. Many of these German Americans had immigrated from northwestern Germany.