[2] In the Nebraska license plate system, Nemaha County is represented by the prefix 44 as it had the forty-fourth-largest number of vehicles registered in the county when the license plate system was established in 1922.

The grounds of modern Indian Cave State Park mark where the county's first community, Saint Deroin, was founded in 1853 by members of the reservation as a trading post on the Missouri River.

When white settlement was permitted in the Kansas–Nebraska Act of 1854, this area was part of Forney County (named for US cabinet member John W. Forney) which stretched along the Little Nemaha River from the Missouri River to west of Lincoln, Nebraska.

[citation needed] Brownville was the biggest city in Nebraska at the time and several firsts occurred in the county including: in 1861 the first state normal school was founded at what today is Peru State College; Daniel Freeman filed the first claim under the Homestead Act of 1862 for land on January 1, 1863, at the Brownville land office.

[citation needed] In 1974 the Cooper Nuclear Station was commissioned south of Brownville.

[citation needed] Nemaha County lies on the eastern edge of Nebraska.

In the county's extreme northeast corner, the border follows the river's former course, lying nearly two miles (3 km) east of the present-day channel.

[4] The county's terrain consists of low rolling hills, sloping eastward to the Missouri River valley.