After working briefly as railroad porter, insurance salesman, he enrolled as a law student at nearby Washington's all-Negro Howard University where he graduated 3 years later at the top of his class. In 1933 he began work with a Baltimore law firm, specializing in civil rights cases, suffering economically as a result of the low pay from such work. His first major victory was assisting Donald Murray who had been refused admission to the University of Maryland Law School because of his race. In time Marshall gained a reputation for assisting the labor unions, was asked by Arthur Spingarn, one of the Jewish founders of the NAACP to join the civil rights organization where he became somewhat a reformer for less bourgeoisie tendencies within the group. Within years he had risen to the prominent position of director-counsel to the NAACP's Legal Defense and Educational Fund.

Over the course of the next decades Marshall traveled extensively and brought numerous cases up before the U.S. Supreme Court. Among his greatest accomplishments was his record of breaking down barriers to Blacks becoming educated at Whites-only colleges. On May 17, 1954, the Supreme Court, in a case known as Brown verses the Board of Education, struck down the separate-but-equal doctrine that had been the law throughout the U.S. since the late 19th Century. Still the battle for desegregation of educational facilities continued for decades.

In October 1961, President John Kennedy appointed Marshal to an interim appointment on the Federal Court of Appeals, Second District. Nearly one year later he was finally confirmed after a bitter Senate fight to the position. In July 1965, then-President Lyndon Johnson appointed him to Solicitor General, the third-ranking position in the U.S. Justice Department. Two years later he was nominated to the U.S. Supreme Court to which he was confirmed in September 1967. His tenure on the court included a number of civil rights gains for Blacks. His legacy remains a debate as he was accused of colleagues on the court of being lazy and inattentive, he suffered from a full-blown drinking problem, and has been exposed for having given confidential information to the racist FBI director J. Edgar Hoover on fellow black civil rights organizers.