Mr. Stokes, shown here in January, served in Congress for 30 years. (Tony Dejak/AP)

Former U.S. representative Louis Stokes, an Ohio Democrat who became his state’s first black congressman and served 30 years representing a portion of Cleveland, where his brother was elected the first African American mayor of a major American city, died Aug. 18 at his home outside Cleveland. He was 90.

His death was announced by his family. Mr. Stokes said last month that he had lung and brain cancer.

The descendants of a slave and sons of a domestic worker, Louis Stokes and his brother, Carl, the younger by two years, grew up in a public housing development in Cleveland and became two of the most noted Ohio politicians of their era.

In 1967, in a campaign that helped change racial politics in the United States, Carl Stokes was elected to the first of two terms as Cleveland mayor. The next year, Louis Stokes, a lawyer who had brought several cases to the U.S. Supreme Court, won the congressional seat that he would hold until his retirement in 1998.

“Lou believed deeply in fairness and the idea that every American should have the same opportunity to succeed,” President Obama said in a statement. “Growing up in Depression-era Cleveland with his mother and brother Carl, Lou triumphed over hardship to become a passionate voice for those less fortunate.”

Mr. Stokes represented a swath of Cleveland, a predominantly black district that he helped create by mounting a legal challenge to the race-based gerrymandering that had made it difficult, if not impossible, for a black candidate to win election to the House of Representatives. In Washington, he helped found the Congressional Black Caucus in 1971, later serving as chairman.

As Mr. Stokes advanced in seniority, he became the first African American to serve on the Appropriations Committee and became the dean of the Ohio delegation. In the 1980s, he chaired the House Ethics Committee and the House Permanent Select Committee on Intelligence, also serving on the committee that investigated the Iran-contra affair during the Reagan administration.

Perhaps his most public role came as chairman of the House Select Committee on Assassinations, convened from 1976 to 1979 to reopen investigations of the killings of President John F. Kennedy in Dallas in 1963 and civil rights leader Martin Luther King Jr. in Memphis in 1968. At the time of the committee hearings and for decades afterward, doubts and questions swirled around the events.

In a high-profile report, the committee found no involvement in Kennedy’s death by the Soviet government, the Cuban government, the CIA or the FBI. But the report also suggested that Kennedy was “probably assassinated as a result of a conspiracy” — a conclusion that diverged from the findings of the Warren Commission, which named Lee Harvey Oswald as the lone gunman — and cited “failures by the CIA and FBI” to provide the Warren Commission “with all relevant evidence and information.”

During the House committee’s inquiry into King’s death, Mr. Stokes questioned James Earl Ray, who had pleaded guilty in 1969 to murdering the civil rights leader but later recanted his confession.

In scenes that The Washington Post described as “straight out of Perry Mason,” Mr. Stokes demonstrated the skills that he had honed in legal practice, marshalling evidence including a laundry ticket and a change-of-address notice to demonstrate that Ray had methodically trailed King en route to Memphis.

The committee concluded that Ray had assassinated King — rejecting his claims of a frame-up — but cited “compelling indications of conspiracy” not fully investigated a decade earlier.

“What we have done now is to come out with certain conclusions that sort of vindicated what the American people have suspected all along — and that was, in both cases, there was a conspiracy,” Mr. Stokes said in 1979.

He returned to the matter of the Kennedy assassination after the release of Oliver Stone’s film “JFK” (1991), which revived long-festering conspiracy theories about the president’s death. After opposing the release of sealed materials related to the case, Mr. Stokes helped enact legislation that established the Assassination Records Review Board, which secured the release of thousands of documents.

Mr. Stokes also pushed for government transparency after the Iran-contra affair, in which U.S. officials clandestinely sold arms to Iran and funneled some of the proceeds to Nicaraguan rebels known as contras. As Intelligence Committee chairman, Mr. Stokes called for greater congressional oversight of covert CIA activities.

“Never again,” he said, “must we hear that an activity of the U.S. government is so sensitive that knowledge of it must be withheld from the U.S. Congress.”

In the 1980s, Mr. Stokes joined a coalition that sought to promote a black candidate for president. His name surfaced among possible candidates, but support coalesced around Jesse Jackson, the civil rights activist and minister, who made his first unsuccessful run for the White House in 1984.

In the House, Mr. Stokes was recognized as one of the chamber’s most forceful advocates for funding of public housing for the poor, once remarking that if not for such government assistance, he and his brother would be “either in jail or dead.”

“We’d be some kind of statistic,” he said.

Louis Stokes was born in Cleveland on Feb. 23, 1925. After his father’s death from an illness at 28, Louis — nicknamed Billy — and Carl lived in a poorly heated home with a single bed. Their mother, Louise, had an eighth-grade education and emphasized schooling, telling her sons to “get something in your head so you don’t have to work with your hands.”

Before pursuing higher education, Mr. Stokes served in the Army during World War II. In an interview with Cleveland magazine, he recalled being separated from white troops and placed with German prisoners of war during a train trip through the segregated South. At a stop for a meal, the prisoners, but not the black servicemen, were permitted to eat in a “whites only” restaurant.

Mr. Stokes received a law degree from Cleveland-Marshall College of Law in 1953 and began his practice in Cleveland. He helped argue Terry v. Ohio, a landmark case in which the Supreme Court ruled in 1968 that police may stop and frisk a suspect without consent if they believe the suspect is engaged in criminal activity and may be armed and dangerous.

In Congress, Mr. Stokes led the Ethics Committee during the Abscam corruption scandal of the early 1980s. He recused himself from the investigation of the congressional banking scandal of the 1990s, in which he was found to have made more than 500 overdrafts.

In 1983, he was stopped by police after driving the wrong way on a Maryland road and driving through a red light at about 2 a.m., after a late session of Congress. Mr. Stokes, who insisted that he was not intoxicated but was suffering from fatigue and an allergy, was acquitted of driving while intoxicated but convicted of the lesser charge of driving under the influence of alcohol. He was fined $250.

Mr. Stokes was defended by then-D.C. Del. Walter E. Fauntroy (D), among others, who accused the police of stopping Mr. Stokes because of his race.

Survivors include his wife of nearly 55 years, the former Jay Francis; four children, Shelley Stokes-Hammond, Lori Stokes, Angela R. Stokes and Chuck Stokes; and seven grandchildren.

Carl Stokes, who after serving as mayor became an anchorman for WNBC in New York and later U.S. ambassador to the Seychelles, died in 1996, nearly three decades after capturing national attention with his electoral victory in Cleveland.

“I realized I had to live with being Carl Stokes’s brother until I could establish my own independent image,” Louis Stokes told the Cleveland Plain Dealer in 1998.

He continued, “Later on in life, Carl got a bang out of the fact that people would come up to him and ask, ‘Are you Lou Stokes’s brother?’ ”

An earlier version of this article incorrectly reported that the House Select Committee on Assassinations was convened from 1977 to 1979. It was convened from 1976 to 1979. The article has been updated.