Mercer County was set apart and established as a separate county, January 2, 1824. During a 61 year period after the Ordinance of 1787 established the Northwest Territory, Mercer County was a part of six different counties. Its first affiliation was as a part of Knox County, which was the narrow area lying west of the Great Miami River from its mouth in the Ohio to a point north of Piqua and extending into what was Indiana Territory. This is not to be confused with what is now Knox County, Ohio.
Mercer County was named in honor of General Hugh Mercer, a hero of the Revolutionary War.
General Arthur St. Clair met his disastrous defeat near the headwaters of the Wabash River in Mercer County, November 4, 1791. His original army of 2000 men, poorly equipped and the majority of the untrained, had decreased to 1400, mainly by desertion. Of this number, about 900 were killed, wounded or missing after being surprised and ambushed by the Indians under their famous chief, Little Turtle.
General Anthony Wayne, the ablest of President Washington’s General, with 5000 well equipped men skilled in Indian warfare, cut their way to the site of the St. Clair defeat, where he routed the Indians and built Fort Recovery. Later he built Fort Adams in the northern part of Mercer County, on the St. Marys River. Leaving a garrison and supplies there, he proceeded to the Maumee River where he defeated the united Indian tribes and saved the Northwest Territory. A replica of old Fort Recovery and its stockade now stands on the original site which, along with a Museum is an excellent tourist attraction.
The first church in Mercer County was built of logs in 1818, near the present site of Mendon. The first school was started in 1824 at Shanesville, now Rockford. The oldest town in the county is Rockford, founded in 1820 and named Shanesville, by the founder Anthony Shane, half French and Indian who operated a trading post on the St. Marys River, prior to 1820.
The ancient Teays River which coursed through the Mid-eastern states more than a million years ago was the forerunner of the present river systems. The Ohio, Muskingum, Scioto, Miami, and the Wabash Rivers were not in existence in that era. The Teays had its source in the mountains of North Carolina and after flowing through West Virginia entered Ohio territory near Huntington, WV. The glaciers of the ice age planed off hills and filled valleys and filled valleys and covered the bed of the Teays to depths from 300 to 400 feet, of stones, gravel, soil and other debris. The drilling of deep water wells throughout Ohio has enabled archeologist to quickly and accurately follow its course through deposits brought to the surface. The Teays passed through Mercer County in a northwesterly direction after entering the eastern boundary.
While very few mounds are found as far north as Mercer County, this fringe area of the domain of the Mound Builders of the Hopewell era contains ten earthworks including enclosures, burial grounds and one mound.
Mercer County was in the territory of the powerful and warlike Miami Indian tribe. Later during the early 1800s, Shawnees moved from southeastern Ohio to the St. Marys River where they lived on Reservations until the Government moved them to a Reservation in Kansas. A lesser number migrated to Missouri, Arkansas, and Oklahoma.
The first explorer to pass through Mercer County was Charles Langlade, a French-Indian, in 1752. He was enroute to Pickawillany (Piqua) where in a surprise attack he dealt British prestige a severe blow. Miami Chief La Demoiselle, friend of the British, was killed.