[3] It was named after John Barnwell, a local figure in the Revolutionary War.

The government was made more democratic, with county officials to be elected by male citizens at least 21 years old,[5] rather than by the state legislature as done previously.

Aiken and Barnwell, with nearly equal, populations of Black and white people, had extensive violence in the months before the 1874 and 1876 elections, as groups of paramilitary Red Shirts rode to disrupt Black Republican meetings and intimidate voters to suppress Black voting.

More than 100 Black men were killed in Aiken County during the violence, especially at Ellenton, South Carolina.

In 1895, white Democrats in the state legislature passed a new constitution, disfranchising most Blacks for more than 60 years by raising barriers to voter registration.

25.60% of all households were made up of individuals, and 10.20% had someone living alone who was 65 years of age or older.

Prior to 1948, Barnwell County was a Democratic Party stronghold similar to the rest of the Solid South, with Democratic presidential candidates receiving near-unanimous margins of victory in most years.

The twenty years from 1948 to 1968 were a highly transitional time for the politics of South Carolina & Barnwell County, largely in part due to the Democratic Party's increasing support for African-American civil rights & enfranchisement.

Segregationist candidates Strom Thurmond & George Wallace won the county in those aforementioned years, bookended by Democratic wins in 1952 & 1956 & Republican wins in 1960 & 1964.

[23] From 2022 through 2024, the unemployment rate for the county has fluctuated from a high of 6.3% in January 2022 and a low of 3.2% in April 2023.

[24] As of April 2024[update], Walmart is one of the largest employers in the county.