Many white people fought alongside African Americans in the civil rights movement. But few made as vital and enduring an impact as Jack Greenberg, a protege of and successor to Thurgood Marshall at the NAACP Legal Defense and Educational Fund.

Mr. Greenberg, who died Oct. 12 at 91, joined the New York-based legal organization in 1949, fresh out of Columbia Law School. At the time, civil rights law was a small field frequently overlooked by ambitious white lawyers, but Mr. Greenberg said he was invigorated by the principles at stake and the intellectual challenge on hand.

He became part of Marshall’s inner circle at the fund, helping argue landmark civil rights cases such as Brown v. Board of Education, resolved in 1954 when a ruling by the U.S. Supreme Court abolished “separate but equal” racially segregated public schools. (Mr. Greenberg was the last living lawyer to argue the case.)

When Marshall left the fund in 1961 for a federal appellate judgeship — later becoming U.S. solicitor general and then, in 1967, the first black Supreme Court justice — he hand-picked Mr. Greenberg as director-counsel of the fund, often called LDF.

Mr. Greenberg spent 23 years at its helm, taking to the high court major cases involving desegregation, employment discrimination and what he lambasted as the racially biased application of the death penalty.

Sen. Ted Kennedy (D-Mass.), right, is seen at a luncheon of the NAACP Legal Defense and Education Fund in New York with NAACP counsel and director of the fund, Jack Greenberg, seated at left, May 15, 1969. With them is the Rev. M. Moran Weston, a member of the board of directors. (Tony Camerano/AP)

Occasionally, he encountered resistance from blacks who resented the high-profile authority of a white person at a legal fund long under the auspices of the National Association for the Advancement of Colored People.

Marshall stamped out any such objections. “As those who are fighting discrimination,” he said, “we cannot afford to practice it.” Mr. Greenberg, Time magazine quoted Marshall as saying, was “about as Negro as a white man can get.”

Mr. Greenberg’s most enduringly important role came early in his career, when he was an assistant counsel to Marshall on school desegregation cases. They and other LDF colleagues sought not merely to obtain equal resources for black schools but to attack the very notion of “separate but equal.”

Mr. Greenberg’s work included finding experts — such as eminent psychologists, sociologists and social scientists — who testified with authority about the damage that segregation, especially legally sanctioned segregation, wreaked on young people.

In Delaware in 1949, he teamed with Louis L. Redding, the first — and for decades the only — black lawyer in the state to represent black students who had been denied admission to the University of Delaware.

They “put together an overwhelming case on the comparative merits of the white university and the Delaware State College for Negroes,” author Richard Kluger wrote in “Simple Justice: The History of Brown v. Board of Education and Black America’s Struggle for Equality” (1975).

In 1950, the Delaware Court of Chancery ordered the desegregation of the University of Delaware. On the heels of that victory, Mr. Greenberg and Redding filed in 1951 a suit on behalf of black children in Delaware who were prohibited from attending white public schools.

Lawyer and legal scholar Jack Greenberg in 2014. (Chip Somodevilla/Getty Images)

The racial disparity in educational opportunity was evident in Claymont, Del., where whites attended a 14-acre, 400-student combination grade school and high school that offered courses in economics, public speaking, Spanish and trigonometry. None of these courses were offered at the 1,300-student Howard High in downtown Wilmington, where black students statewide were bused on trips lasting as long as an hour-plus each way.

A Delaware judge ruled that black children were receiving an inferior education and ordered that they be admitted to white schools, but he declined to strike down the “separate but equal” doctrine, finding that such a decision rested with the U.S. Supreme Court.

Ultimately, the Delaware public schools case, along with four others from Virginia, South Carolina, Kansas and the District of Columbia, reached the Supreme Court under the umbrella of Brown v. Board.

Evidence presented at the Delaware trial court formed the underpinning for Mr. Greenberg’s oral arguments and the legal briefs he helped prepare for the high court.

“All the expert testimony really said was that if you stopped and thought about it, segregated people are clearly the worse off for it. Yes, it’s hard to demonstrate this precisely, but doesn’t common sense tell you as much?” he told Kluger. “The main function of the social-science testimony was to help the courts — and especially the Supreme Court — convey the confidence of its common-sense perceptions of what the nation knew about right and wrong in this regard.”

LDF became a separate entity from the NAACP in 1957, and Mr. Greenberg took over four years later.

Citing accolades from colleagues, Kluger praised the lawyer’s “supple and uncluttered mind, great intellectual energy eagerly exercised and methodically disciplined, the courage to take a position on a complex legal question and the stamina to stick to it, and the manipulative skills to keep a large organization of professionals working with dedication toward a goal beyond their own enshrinement.”

Despite the decision in Brown v. Board, political and social resistance to integration remained strong in the South and elsewhere. Mr. Greenberg led a modestly staffed and budgeted office that plunged into hundreds of civil rights cases.

He helped litigate Meredith v. Fair, which resulted in James Meredith’s 1962 integration of the University of Mississippi. The next year, the Rev. Martin Luther King Jr., the civil rights leader who co-founded the Southern Christian Leadership Conference, enlisted the fund to handle discrimination cases.

On March 16, 1965, LDF lawyers including Mr. Greenberg filed a proposed plan for a second voting rights march from Selma, Ala., to the state capital of Montgomery after authorities earlier that month had attacked civil rights protesters on the Edmund Pettus Bridge.

Their plan for the five-day march — detailing the precise route to be followed, the food, first aid and other services to be provided, and at what point songs would be sung — was approved by a federal judge, establishing their right to hold the demonstration without disruption.

Tens of thousands of people participated in the march, which was credited with helping propel congressional approval that summer for the Voting Rights Act, which prohibited racial discrimination at the polls.

In a SCOTUSblog podcast years later, Mr. Greenberg said the legal fund’s persistent battle was “little by little by little . . . persuading the Supreme Court to sort of tighten the screws somewhat and the lower courts to give some real meaning to Brown. And we won some cases which essentially closed off the various avenues of escape.”

He argued Alexander v. Holmes County (Miss.) Board of Education, which came to the U.S. Supreme Court in 1969 after a powerful U.S. senator, John C. Stennis (D-Miss.), among others, tried to delay school integration. In that case, 15 years after the ruling in Brown, the high court ordered that segregation end “at once.”

Influenced at Columbia Law

James Jacob Greenberg was born Dec. 22, 1924, in New York City, and he grew up in Brooklyn and the Bronx. His Jewish immigrant parents had fled the pogroms and anti-Semitism of Eastern Europe. His father was a certified public accountant.

In 1945, Mr. Greenberg graduated from Columbia, in absentia, while serving in the Navy at the end of World War II, including in the battles of Iwo Jima and Okinawa. He then enrolled at Columbia Law School, where he was influenced by a class on civil rights and liberties taught by Walter Gellhorn.

Mr. Greenberg completed his law degree in 1948 and joined LDF after Gellhorn recommended him to Marshall. Gellhorn once told Kluger that it was Mr. Greenberg’s emotional reserve, more than his passion for causes, that made him seem suited to the crucible of civil rights work.

“He doesn’t get into a flap when the going gets rough,” Gellhorn said, “and he can stand controversy without developing animosity. . . . This coolness of his — which Thurgood Marshall shared, by the way — was tactically correct, whether conscious or not. After all, they were asserting legal rights — not simply arguing moral positions, and they took as intellectual an approach as possible. An emotional one would not have been productive, especially in the South.”

After the civil rights era, Mr. Greenberg expanded his legal work to racial discrimination in employment and the death penalty.

One matter involved a precedent-setting 1971 Supreme Court case, Griggs v. Duke Power Co. (a utility operating in the Carolinas). In Griggs, the court unanimously struck down tests and other employment and promotion barriers that “are fair in form but discriminatory in operation.”

Mr. Greenberg also helped argue Furman v. Georgia, a 1972 Supreme Court case that drew attention to what LDF lawyers said in part was the capricious application of the death penalty, including an inherent racial bias. The court invalidated existing death-penalty laws, only to reinstate them four years later in another ruling after many states issued revised sentencing laws.

In a 1986 essay for the Harvard Law Review Association, Mr. Greenberg explained his continuing opposition to capital punishment: “Since at least 1967, the death penalty has been inflicted only rarely, erratically, and often upon the least odious killers, while many of the most heinous criminals have escaped execution.

“Moreover, he added, “it has been used almost exclusively against killers of whites, not blacks. It is this system, not some idealized one, that must be defended in any national debate on the death penalty.”

After retiring from the LDF, Mr. Greenberg became vice dean of Columbia Law School and served as dean of the undergraduate Columbia College from 1989 to 1993.

His books included “Race Relations and American Law” (1959), which future New York Times columnist Anthony Lewis hailed as “an indispensable” look at segregation; “Crusaders in the Courts” (1994), about Mr. Greenberg’s work at the legal fund; and “Brown v. Board of Education: Witness to a Landmark Decision” (2004).

Mr. Greenberg’s first marriage, to Sema Tanzer, ended in divorce. Survivors include his wife, Deborah Cole, whom he married in 1970; three children from his first marriage, Sarah Greenberg and Ezra Greenberg, both of Manhattan, and David Greenberg of Oceanside, Ore.; two stepchildren he adopted, Suzanne Greenberg of Manhattan and William Cole of Sitges, Spain; a brother, Daniel Greenberg of Washington; and five grandchildren. A son from his first marriage, Josiah Greenberg, died in 2011 after being struck by a New Jersey Transit train.

Mr. Greenberg died at his home in Manhattan, his wife said. He had Parkinson’s disease, but that was not the cause, she said.

In 2001, President Bill Clinton awarded Mr. Greenberg a Presidential Citizens Medal, calling him “a crusader for freedom and equality for more than half a century.”

Of going to work for Marshall as a young man, Mr. Greenberg told Kluger: “The question of race never really entered into it. It was a question of human liberty. It was the principles that were involved.”