[1] He was involved in numerous crucial cases, including Brown v. Board of Education, which ended segregation in public schools.

[10] During World War II, Greenberg served in the United States Navy and fought at Okinawa and Iwo Jima.

[12] During his service, he was disturbed by racial prejudice he perceived in the Navy, and was threatened with a court martial for shouting at a superior officer in defense of a black crewman that he felt was being mistreated.

Brown declared state laws establishing separate public schools for black and white students to be unconstitutional.

In Brown, Greenberg found social scientists and other authorities from the fields of psychology and sociology who addressed the detrimental effects forced segregation could have on young public school students.

[18] Other civil rights cases Greenberg argued include Alexander v. Holmes County Board of Education in 1969, which ordered the end of segregated school systems "at once", and Griggs v. Duke Power Company in 1971,[19] which outlawed basing employment and promotion decisions on the results of tests with a discriminatory impact.

[15] In 1972, he argued Furman v. Georgia (1972),[20][21] in which the Court held that the death penalty as it was then applied was a violation of the "cruel and unusual punishment" clause of the Eighth Amendment.

Greenberg was a founding member of the Mexican American Legal Defense and Education Fund (MALDEF) and of Human Rights Watch.