In 1938 Marshall succeeded Houston as national special counsel, responsible for all cases involving citizenship rights for blacks. In 1950, Marshall was named director-counsel of the NAACP's Legal Defense and Educational Fund, where he spearheaded the elimination of practices and laws that prevented blacks from enjoying their full rights of citizenship. Among his greatest triumphs was the 1954 United States Supreme Court case Brown v. the Board of Education, in which he and his associates successfully challenged the legality of racial segregation in the nation's public schools.

Appointed to the United States Court of Appeals in 1962, Marshall later served as the nation's first African American solicitor general. In 1967 he became the first African American to sit on the Supreme Court, where he distinguished himself as a spokesman for racial equality and the rights of the underprivileged. Always modest when it came to talking about his achievements, Marshall observed, at his retirement in 1991, that he wanted to be remembered simply as a man who had done "what he could with what he had."