The area of Ohio was part of the original territory of the United States, being part of lands ceded by four states to the United States and designated in 1787 as the "Territory northwest of the River Ohio." It also included the area known as the Connecticut Western Reserve, which was not ceded by Connecticut to the United States until 1800. Ohio Territory was organized on April 30, 1802, from the Northwest Territory, with generally the same boundary as the present state. Ohio was admitted to the Union on February 19, 1803, as the 17th state. Ohio acquired legal title to the area known as the Toledo Strip when Michigan ceded its claim in 1836.

Although not yet legally established as a separate entity, census data for Ohio are available beginning with the 1800 census. The 1800 census population for Michigan includes the population of a small area in present-day Ohio. The 1800 population of the legally established Northwest Territory (of which Ohio was a part) was 45,916. For an explanation of the revision to the 1800 population of Ohio, see Richard L. Forstall, Population of States and Counties of the United States: 1790-1990, Washington, DC: U.S. Government Printing Office, 1996, page 126.

Data for the legally established state of Ohio are available beginning with the 1810 census.