<P> 13 counties were formed by Governor Arthur St. Clair during the territory's existence: </P> <Ul> <Li> Washington County, with its seat at Marietta, was the first county formed in the territory, proclaimed on July 26, 1788 by territorial governor St. Clair . Its original boundaries were proclaimed as all of present - day Ohio east of a line extending due south from the mouth of the Cuyahoga River, but this did not take into account Connecticut's still unresolved claim of the Western Reserve . It kept these boundaries until 1796 . </Li> <Li> Hamilton County, with its seat at Cincinnati, was proclaimed on January 2, 1790 . The same proclamation officially changed Cincinnati's name from Losantiville into its present form . Its original boundaries claimed all land north of the Ohio between the Great Miami River and Little Miami River as far north as Standing Stone Fork (now Loramie Creek), just north of present - day Piqua . In 1792 Hamilton County was expanded to encompass all lands between the mouths of the Great Miami and Cuyahoga Rivers, as well as all of what is now the Lower Peninsula of Michigan . Its territory was reduced several times after 1796 . </Li> <Li> St. Clair County, with its seat at Kaskaskia was proclaimed on April 27, 1790 . It originally encompassed most of present - day Illinois south of the Illinois River . It lost most of its southern lands in the formation of Randolph County in 1795, necessitating the transfer of the county seat to Cahokia, but would expand to the north to take in northwest present - day Illinois and most of present - day Wisconsin in 1801 after becoming part of Indiana Territory . </Li> <Li> Knox County, with its seat at Vincennes, was proclaimed on June 20, 1790, and encompassed the majority of the territory's land area--all land between St. Clair County and Hamilton County, extending north to Canada . </Li> <Li> Randolph County was formed October 5, 1795 with its seat at Kaskaskia and encompassed the southern half of what was St. Clair County . </Li> <Li> Wayne County was formed on August 15, 1796, out of portions of Hamilton County and unorganized land, with its seat at Detroit, which had been evacuated by the British five weeks previously . Wayne County originally covered all of Michigan's Lower Peninsula, northwestern Ohio, northern Indiana and a small portion of the present Lake Michigan shoreline, including the site of present - day Chicago . The lands west of the extension of the present Indiana - Ohio border became part of Indiana Territory in 1800; the eastern portion of the county's land in Ohio were folded into Trumbull County that same year . The territory north of the Ordinance Line became part of Indiana Territory in 1803 as a reorganized Wayne County; the remainder reverted to unorganized status after Ohio statehood . </Li> <Li> Adams County was formed on July 10, 1797, with its seat at Manchester; it encompassed most of present - day south central Ohio . </Li> <Li> Jefferson County was formed July 29, 1797 with its seat at Steubenville, carved out of Washington County and originally encompassed all of what is now northeastern Ohio . </Li> <Li> Ross County was organized on August 20, 1798 with its seat at Chillicothe and was carved out of portions of Knox, Hamilton and Washington counties . </Li> </Ul> <Li> Washington County, with its seat at Marietta, was the first county formed in the territory, proclaimed on July 26, 1788 by territorial governor St. Clair . Its original boundaries were proclaimed as all of present - day Ohio east of a line extending due south from the mouth of the Cuyahoga River, but this did not take into account Connecticut's still unresolved claim of the Western Reserve . It kept these boundaries until 1796 . </Li> <Li> Hamilton County, with its seat at Cincinnati, was proclaimed on January 2, 1790 . The same proclamation officially changed Cincinnati's name from Losantiville into its present form . Its original boundaries claimed all land north of the Ohio between the Great Miami River and Little Miami River as far north as Standing Stone Fork (now Loramie Creek), just north of present - day Piqua . In 1792 Hamilton County was expanded to encompass all lands between the mouths of the Great Miami and Cuyahoga Rivers, as well as all of what is now the Lower Peninsula of Michigan . Its territory was reduced several times after 1796 . </Li>

The first law of the land after the revolutionary war