In the Nebraska license plate system, Fillmore County is represented by the prefix 34 (it had the thirty-fourth largest number of vehicles registered in the county when the license plate system was established in 1922).

Fillmore County was established, and its boundaries defined, by the Nebraska Territorial Legislature in 1856.

It was named for Millard Fillmore, the thirteenth president of the United States, who had left office in 1853.

[8] In 1870, Fillmore City, the county's first town, was established on the Big Blue about four miles (6 km) north of present-day Grafton;[8][9] in 1871, the county's first post office opened in Fillmore City.

A recent change in federal law allowed them to alter their route, shifting it about five miles (8 km) south of their original surveyed path.

[10][12][13] The legislature passed a bill in the summer of 1872 to allow sale of the school lands.

[10] Its growth suffered a check in the mid-1870s, due to the worldwide depression following the Panic of 1873 and an infestation of grasshoppers in the late summer of 1874.

In about 1886, the Burlington built a branch line from Beatrice to Holdrege, running east–west through southern Fillmore County; Shickley, Strang, and Ohiowa were founded on or near the route.

The Big Blue and its tributaries have incised channels into the loess surface in places, but in much of the county the original plain remains.

These loess-plain regions are characterized by extensive upland flats with shallow depressions, lined with fine-grained and relatively impermeable silt, and tend to form shallow ephemeral wetlands when filled with rain or snowmelt; such wetlands range in area from less than 40 acres (16 ha) to more than 500 acres (200 ha).

[28] The county's surface is underlain by Cretaceous sedimentary bedrock, topped with unconsolidated Quaternary sediments.

The bedrock was eroded into hills and valleys before the deposition of the overlying sediments, so the thickness of the latter varies.