Mercer County, named for Brig. Gen. Hugh Mercer, a Scottish surgeon and command during the Revolution, was formed March 12, 1800, from Allegheny County and organized in February 1804. The county encompasses 670 square miles, bordering the Ohio line for 32 miles.

Indian trails that traversed the county included the Cayahaga Path from Franklin to Akron, which entered the county near Sandy Lake, followed its north shore to Pymatuning (now flooded by the Shenango Reservoir), one-and- a-half miles west of Big Bend, and turned Southwest to West Middletown, following Pa. 318 to the state line. The Kuskusky- Cussewago Path ran by Pulaski, West Middlesex, Clark and Greenville, thence northwest to the county line.

The Wyandot and Delaware Indian village, Shenango Town, was situated on the river bank near West Middlesex in 1750-85 and was under the control of the Seneca Indians, of whom a few remained in the area until 1812. Pymatuning Delaware Indian Town, at Big Bend, was located in 1764 to 1785 on the Shanango River at about Clark.

The Beaver and Erie Canal traversed the county, following the course of the Shenango River north to Greenville and thence to the Crawford County line via little Crooked Creek Valley.

The county had 'depreciation lands' and 'donation lands." Benjamin Stokely, deputy surveyor for the county, surveyed some of these lands in 1785. The Holland Land Company and the Pennsylvania Population Company, among others, were active in the region. The county had few settlements until after 1795 when Anthony Wayne signed the Treaty of Greenville with the Indians.

Many of the first settlers were Scottish Presbyterians. Others came from Washington County and introduced sheep raising, with the county ranking third in the state in wool growing as late as 1868. William McMillan, one of three trustees appointed to conserve the assets of the new county and son of Rev. John McMillan, erected a blockhouse in Coolspring Township and built several area churches. In 1885 a large number of Italians settled in Sharon and Farrell when a pipeline was laid from the Butler and Venango County oil fields through Sharon to Youngstown, Ohio.

Among the prominent residents were James Pierce of Hickory Township, who helped develop the bituminous coal industry in the mid 1800s; Jonathan Dunham, who settled in Sharpsville in 1798; and James Sharp, an original landowner of the area. Benjamin Bentley came to Sharon in 1803, and William Budd laid it out in 1819. Alfred Landon, governor of Kansas and Republican candidate for president, was born in West Middlesex