[2] The Georgia General Assembly proposed the constitutional amendment to create the county on August 17, 1920, and the citizens of the state voted in favor of the amendment on November 2, 1920.

Lamar County was named after Confederate Democrat Lucius Quintus Cincinnatus Lamar II.

The western third of Lamar County, west of a line from Orchard Hill through Milner and Barnesville, is located in the Upper Flint River sub-basin of the ACF River Basin (Apalachicola-Chattahoochee-Flint River Basin).

The eastern majority of the county is located in the Upper Ocmulgee River sub-basin of the Altamaha River basin.

[6] As of the 2020 United States census, there were 18,500 people, 6,494 households, and 3,746 families residing in the county.