“I told him quite candidly,” Mr. Prettyman recalled in 1996 in an oral history interview with the Historical Society of the District of Columbia Circuit, “that it sounded more like a dissent than a concurring opinion.”

After Chief Justice Fred M. Vinson died in 1953, his successor, Earl Warren, presented Jackson with a draft of what would become the final Brown decision. After reviewing it, Mr. Prettyman, by his account, told Jackson, “You know, it meets a lot of the problems that you had, as expressed in your unpublished opinion, and while it certainly doesn’t contain a lot of law, it makes sense, it hangs together, it doesn’t offend people, it reads well, anybody can understand it.”

Jackson, he said, replied, “Exactly.”

On May 17, 1954, the court ruled unequivocally that, “in the field of public education, the doctrine of ‘separate but equal’ has no place.”

In his book, Mr. Kluger concluded: “It is doubtful if any of the many excellent young men who have come fresh out of the law schools or soon thereafter to serve the justices of the Supreme Court ever served more faithfully or usefully than Barrett Prettyman served Robert Jackson. What part Prettyman’s memo played will never be known, but it is a fact that Jackson, having written this much on the segregation cases, wrote no more.”

In 1962, Mr. Prettyman, a lifelong Democrat, was recruited to help the Kennedy administration, covertly, to speed the donation and shipment of more than $50 million in agricultural and medical supplies to Cuba as ransom for more than 1,000 hapless Cuban exiles who had been captured the year before while trying to retake their homeland from Fidel Castro.

Unbeknown to Mr. Castro, the supplies included surplus products that American companies were dumping as a way to take tax deductions. To prevent Mr. Castro from immediately finding out, Mr. Prettyman flew to Havana and persuaded him to accompany him for the day on a visit to Ernest Hemingway’s old house outside Havana. By the time they returned to the docks, all Mr. Castro could see was a bountiful supply of baby food.

“I did get in the fact that because of the way we had to load, he might be getting some things he could throw away, but by then he was sufficiently pleased with what he’d seen,” Mr. Prettyman said, “and he ordered that the prisoners could start to leave right away.”