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

The United States acquired the area of Nebraska from France as part of the Louisiana Purchase in 1803. Nebraska Territory was organized on May 30, 1854, from the northwestern part of Missouri Territory (also called Indian Country or Indian Territory). At the time of its organization, the territory included almost all of present-day Nebraska and parts of Colorado, Montana, North Dakota, and South Dakota. The area of the territory was reduced greatly in 1861 with the organization of Colorado and Dakota territories. In 1882, Nebraska acquired from Dakota Territory an area north of the Keya Paha River and Niobrara River to assume generally the same boundary as the present state. Nebraska and Iowa have made periodic adjustments to their boundary as the Missouri River has changed its course.

Census data for Nebraska are available beginning with the 1860 census. The 1860 population is for part of the area of the Nebraska Territory including all of present-day Nebraska, and settlements now within Colorado, South Dakota, and Wyoming. If the part of Nebraska Territory in 1860 that is in present-day Montana and North Dakota, as well as additional parts of South Dakota, was enumerated, the population was included as part of unorganized Dakota; thus, no accurate data are available for the entire Nebraska Territory as legally defined.

Data for the legally established state of Nebraska are available beginning with the 1870 census.