Preble County was formed March 1, 1808, from parts of Butler and Montgomery counties. It was named for Capt. Edward Preble, a naval hero of the American Revolution and the Tripolitan War. Preble, born Aug. 15, 1761, in Maine, died June 25, 1806, less than two years before the formation of the county. From 1803 until action by the Ohio General Assembly Feb. 15, 1808, the southern part of what became Preble County was in Butler County and the remainder in Montgomery County. Fort St. Clair was built in 1792 as a supply post during the Indian Wars, 1790-1795.Major John S. Gano directed its construction. The fort, west of the future location of Eaton, was the site of a skirmish Nov. 6, 1792, between Kentucky militiamen and Indian. Settlement began after Gen. Anthony Wayne's defeat of the Indians in the battle of Fallen Timbers Aug. 20, 1794, and completion of the Treaty of Greenville in 1795.
The first settlement in the county was by John Leslie in 1798 along Elk Creek, in what is now known as Gratis Twp. (in the southeast corner of the county, abutting Butler County to the south). Laid out in 1805, New Lexington (now in Twin Twp., Ohio 503 and Lexington Road) was the first town in the county. Eaton followed, platted during the winter of 1805-1806 by William Bruce, who chose a site along Seven Mile Creek for the location of his mill. The town was named for Gen. William Eaton, who distinguished himself in the Tripolitan War in 1805.
There are 12 townships in Preble County with three having Butler County as their southern neighbor -- Gratis on the east, Sommers in the middle and Israel on west with Indiana on its western boundary. Other townships are Dixon, Gasper, Lanier, Jackson, Washington, Twin, Jefferson, Monroe and Harrison.