“He was a natural,” Ms. Sobol said in an interview. “He practiced law on a whole different level from most of us.”

Mr. Sobol often said that his greatest defeat was his failure to convince the Supreme Court in 1972 that juries should be required to reach unanimous decisions. The court revisited the issue recently and, in a triumph that he did not live to see, ruled on Monday that jury decisions involving serious crimes had to be unanimous.

Mr. Sobol practiced primarily in Louisiana and Washington, D.C. But he preferred working in the trenches in Louisiana than on antitrust cases for the white shoe firm in Washington that employed him. In a description of his early career — which he wrote as a chapter for “Voices of Civil Rights Lawyers” (2017), edited by Kent Spriggs — he said that most of his work in Washington “never came to anything, certainly not to anything one could be proud of.”

By contrast, he wrote, within 10 days of arriving in Louisiana in 1965, he won a school desegregation case that allowed black children to attend white schools. “I saw the impact one lawyer, familiar with federal litigation practice, could have,” he wrote.

He stayed in Louisiana longer than he had initially planned. And across the decades he made a difference in scores of cases, big and small.

“He devoted his life to seeing that justice was done,” George Cooper, a retired professor from Columbia Law School, who met Mr. Sobol in the early 1960s and worked on cases with him, said in a phone interview.

“He was one of the legions of young lawyers who went South in the 1960s to help with the civil rights movement,” Mr. Cooper said. “But unlike so many others, he stayed on the ground and saw it through. In the process, he won notable cases but also gave a whole segment of the population a chance for justice that they might not have had otherwise.”