After law school, Marsh joined with Samuel W. Tucker to form the law firm of Tucker & Marsh in 1961. They were joined by Hill in 1965 to form Hill, Tucker & Marsh. Marsh concentrated on civil rights law, participating in such cases as "Quarles v. Philip Morris", the first U.S. legal case involving racial discrimination in employment, which set the precedent for prohibiting department seniority systems and requiring equal pay for equal work. After that, he successfully litigated more than 20 "employment discrimination cases, most of which were class-action cases, representing thousands of African American and female litigants."