It took 10 minutes to convict 14-year-old George Stinney Jr. It took 70 years after his execution to exonerate him.

In March 1944, deep in the Jim Crow South, police came for 14-year-old George Stinney Jr. His parents weren’t at home. His little sister was hiding in the family’s chicken coop behind the house in Alcolu, a segregated mill town in South Carolina, while officers handcuffed George and his older brother, Johnnie, and took them away.

Two young white girls had been found brutally murdered, beaten over the head with a railroad spike and dumped in a water-logged ditch. He and his little sister, who were black, were said to...