1846), which was named after the granite strata that rests under the county's red clay top soil.

Prior to Rockdale becoming a county, the land had been inhabited by the Creek and Cherokee; the boundary between the two native nations, the Hightower Trail, ran directly through the area.

During the American Civil War, General William Tecumseh Sherman marched the Union Army north of Conyers on his way to Covington from Lithonia.

According to a historical marker on U.S. Highway 278 west of Conyers, Major General Joseph Wheeler of the Confederate States Army and part of his staff were captured by Union troops pursuing Jefferson Davis on May 9, 1865.

A former Republican stronghold, Rockdale County has undergone a massive shift towards the Democratic Party in recent decades, primarily due to large growth of the African-American population.

In 2000, Republican presidential nominee George W. Bush won 62.64% of the county's vote, defeating Democrat Al Gore by 29 percentage points.

However, just 20 years later, Democratic nominee Joe Biden won 69.88% of the county's vote, defeating Republican incumbent Donald Trump by nearly 41 percentage points.

Contrary to national and statewide trends, Rockdale County delivered an even larger margin for Democratic nominee Kamala Harris in 2024, who defeated Republican nominee Donald Trump by 47 percentage points even as Trump narrowly flipped the state of Georgia back to the GOP column.