In 1868, under the South Carolina Constitution revised during Reconstruction, South Carolina districts became counties. The government was made more democratic, with county officials to be elected by male citizens at least 21 years old, rather than by the state legislature as done previously.
In 1871 the legislature took the northwestern portion of the county to form part of the new Aiken County, the only county organized during the Reconstruction era. In 1874 the border with Aiken County was adjusted slightly. This county and Barnwell, with populations of blacks and whites that were nearly equal, 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.
In 1897 the eastern third of the county was taken to form the new Bamberg County. In 1919 most of the southern half of the county was taken to form most of the new Allendale County, thus reducing Barnwell county to its present size.
Aiken County - north
Bamberg County - east
Orangeburg County - east
Allendale County - southeast
Burke County, Georgia - southwest