From the Guide to 2010 State and Local Census Geography – Michigan – History:

The area of Michigan 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." Michigan Territory was organized on June 30, 1805, from the northeastern part of Indiana Territory, and included all of the Lower Peninsula, the eastern part of the Upper Peninsula, a small strip of northern Indiana, and a portion of northwestern Ohio that was later contested. In 1818, when Illinois was admitted as a state, all of the remainder of Illinois Territory was added to Michigan Territory, including almost all of present-day Wisconsin, part of Minnesota, and the western part of the Upper Peninsula; at the same time, the central portion of the Upper Peninsula and eastern Wisconsin were added from the former Indiana Territory. In 1834, Michigan Territory was enlarged from part of Missouri Territory, including all of present-day Iowa, the remainder of Minnesota, and the eastern part of North Dakota and South Dakota. Michigan was reduced with the organization of Wisconsin Territory and the cession of the Toledo Strip to Ohio in 1836. As a compromise for the cession of territory to Ohio, Michigan Territory retained all of the Upper Peninsula when Wisconsin Territory was organized, resulting in a boundary generally the same as the present state.

Although not yet legally established as a separate territory, census data for Michigan are available beginning with the 1800 census. The 1800 census includes the population of a small area that is in present-day Ohio and excludes the population that was enumerated with Indiana of a small area of present-day Michigan. The 1800 population of the legally established Northwest Territory (of which Michigan was a part) was 45,916. The 1810, 1820, and 1830 census populations are for the entire area of Michigan Territory, including population in extensive areas not in the present state, and, in 1810, excluding again the population of a small area of present-day Michigan enumerated with Indiana. For an explanation of the revision to the 1800, 1820, and 1830 population of Michigan, see Richard L. Forstall, Population of States and Counties of the United States: 1790- 1990, Washington, DC: U.S. Government Printing Office, 1996, page 82.

Data for the legally established state of Michigan are available beginning with the 1840 census.