E. Clinton Bamberger Jr., right, and R. Sargent Shriver announce the Houston Legal Services Program in 1966. Mr. Bamberger devoted much of his career to leveling the legal playing field for the poor. (Clinton Bamberger Papers/National Equal Justice Library, Georgetown Law Library)

E. Clinton Bamberger Jr., who as the lawyer for a death row inmate convinced the Supreme Court that prosecutors may not deny exculpatory evidence to a defendant, a seminal ruling in criminal law, died Feb. 12 in Baltimore. He was 90.

He had Alzheimer’s disease and pneumonia, said his son, Edward C. Bamberger III.

Mr. Bamberger began his legal work at Piper & Marbury, one of the most distinguished law firms in Baltimore, where he made partner. He also made “more money than I could spend,” he once quipped, before undergoing what he described as “some kind of midlife crisis.”

He dedicated much of the rest of his career to leveling what is often regarded as the uneven playing field of the law, where only the wealthy have access to the highest-caliber, and most highly paid, lawyers. In 1965, he became the first director of legal services in the Office of Economic Opportunity, led by R. Sargent Shriver. The office was a cornerstone of President Lyndon B. Johnson’s War on Poverty.

Mr. Bamberger later served as dean of Catholic University’s law school, as an executive at Legal Services Corp. — a major funder of civil legal assistance for low-income clients — and as a professor at Harvard University and the University of Maryland, where he coached students in providing legal counsel to the poor. For years, he fought to protect renters from the health dangers of lead-based paint.

But he made perhaps his most enduring contribution to the law as the attorney for John Leo Brady, a Maryland man who was 25 when he was convicted in 1958 and sentenced to death for the murder of a 53-year-old acquaintance, William Brooks, in the course of stealing the victim’s car. An accomplice, Charles D. Boblit, 24, was convicted of murder in a separate trial.

Mr. Bamberger was referred to Brady by a friend, a Jesuit chaplain at the Maryland penitentiary where Brady awaited execution. The chaplain “never met a guilty man,” Mr. Bamberger recalled in an oral history for the National Equal Justice Library at the Georgetown University Law Library.

Brady claimed that he and Boblit had stolen the victim’s vehicle for an aborted bank robbery but that they had not planned to kill him. Brady further insisted that it was Boblit, not he, who had strangled Brooks. Researching the case, Mr. Bamberger found that Boblit had given five statements to law enforcement officials — but that only four of them had been admitted into evidence in Brady’s trial.

“I was curious about that,” Mr. Bamberger said in the oral history, “because in every one of these statements the other guy claimed that Brady did it, actually killed the man. But there were little weaknesses and inconsistencies in them. . . . Anyway, I got ahold of the fifth statement, and in the fifth statement the other man admitted that he was the man who was the one who actually garroted Mr. Brooks.”

Mr. Bamberger appealed Brady’s conviction to a higher court, which denied him a new trial but granted a new hearing on whether he should be sentenced to execution or life imprisonment. Mr. Bamberger, hoping for a new trial, appealed to the Supreme Court.

In 1963, the Supreme Court handed down a 7-to-2 ruling agreeing with the lower court that Brady should receive a new sentencing hearing but not a new trial. But in its decision, the justices formulated what became known as the Brady rule.

“We now hold that the suppression by the prosecution of evidence favorable to an accused upon request violates due process where the evidence is material either to guilt or to punishment. . . . Society wins not only when the guilty are convicted, but when criminal trials are fair,” Justice William O. Douglas wrote.

The Supreme Court granted Brady a new hearing on his sentence, which was ultimately commuted to life imprisonment. He did not receive a new trial.

“Everybody thinks I won it,” Mr. Bamberger told the Texas Tribune in 2013. “They did not reverse the court, but in the course of writing the opinion, they wrote the Brady rule.”

Edward Clinton Bamberger Jr. was born in Baltimore on July 2, 1926. His father was a financial adviser, and his mother was a homemaker.

Mr. Bamberger received a bachelor’s degree in 1949 from what is now Loyola University Maryland in Baltimore and a law degree from Georgetown University in 1951.

His wife of 65 years, the former Katharine Kelehar , died in December. Their daughter, Christine Bamberger, died in 1998. Besides their son, of Timonium, Md., survivors include three grandchildren.

Brady was ultimately paroled. Testifying before the Senate Judiciary Committee in 2012 on the significance of the Brady rule, Carol Brook, a federal defender, recalled what became of the defendant whose name was enshrined in American jurisprudence.

“John Leo Brady . . . moved to Florida and became a truck driver. He started a family and was never in trouble again,” she said. “When his son was old enough to understand, he explained to him what he had done and what happened in his case. Shortly after that, his son sought out the telephone number of his father’s lawyer, Clinton Bamberger, and called him. What he said to Mr. Bamberger was, ‘Thank you for saving my father’s life.’ ”