In the late 1940s and early 1950s, Marshall was the chief staff lawyer for the National Association for the Advancement of Colored People (NAACP). Marshall and the NAACP were trying to change the current law, which said it was OK to have separate schools for blacks and whites as long as they were equal. In reality, separate almost never meant equal.

In 1951, Clarendon County, South Carolina, spent more than three times as much on each white child's education as it did on each black child. A black Clarendon County principal asked black parents to file a lawsuit to demand better schools for their children. The case became known as Briggs v. Elliott, and Marshall led the team of NAACP lawyers.