Ross County was formed on August 20, 1798 by proclamation of Arthur St. Clair, Governor of the Northwest Territory. It included a large part of what is now the State of Ohio. The establishment of Franklin County and the later organization of other counties reduced Ross County’s huge size in 1803. Ross County was named for James Ross of Allegheny County, Pennsylvania, a close friend of Governor St. Clair, although many thought it should be named after Nathaniel Massie who surveyed the area.
In 1800, the capital of the Northwest Territory was moved to Chillicothe, the present county seat. Chillicothe also served as the first capital of the State of Ohio from 1803 until 1816, except for an interval from late 1809 to 1812 when the state legislature met in Zanesville. In 1817, the state capital moved 40 miles north of Chillicothe to Columbus, the state's present capital city, because it was more centrally located.