In Murray v. Pearson, Marshall represented Donald Gaines Murray, a black Amherst College graduate with excellent credentials, who was denied admission to the University of Maryland Law School because of its segregation policy. Black students in Maryland wanting to study law had to attend segregated establishments, Morgan College, the Princess Anne Academy, or out-of-state black institutions. Using the strategy developed by Nathan Margold, Marshall argued that Maryland's segregation policy violated the "separate but equal" doctrine of Plessy v. Ferguson because the state did not provide a comparable educational opportunity at a state-run black institution. The Maryland Court of Appeals ruled against the state of Maryland and its Attorney General, who represented the University of Maryland, stating, "Compliance with the Constitution cannot be deferred at the will of the state. Whatever system is adopted for legal education must furnish equality of treatment now."
At the age of 32, Marshall won U.S. Supreme Court case Chambers v. Florida, 309 U.S. 227 (1940). That same year, he founded and became the executive director of the NAACP Legal Defense and Educational Fund.As the head of the Legal Defense Fund, he argued many other civil rights cases before the Supreme Court, most of them successfully, including Smith v. Allwright, 321 U.S. 649 (1944); Shelley v. Kraemer, 334 U.S. 1 (1948); Sweatt v. Painter, 339 U.S. 629 (1950); and McLaurin v. Oklahoma State Regents, 339 U.S. 637 (1950). His most famous case as a lawyer was Brown v. Board of Education of Topeka, 347 U.S. 483 (1954), the case in which the Supreme Court ruled that "separate but equal" public education, as established by Plessy v. Ferguson, was not applicable to public education because it could never be truly equal. In total, Marshall won 29 out of the 32 cases he argued before the Supreme Court.