His willingness to ask questions of people in power carried over to his professional life and led to his role in the development of the Freedom of Information Act, the landmark legislation providing citizens with a tool to keep the government open and honest.

Mr. Schlefer, a former Washington resident, died Nov. 2 at his home in Putney, Vt. The cause was cardiorespiratory arrest, said a daughter, Katharine Schlefer Dodge. He was 98.

In the early 1960s, while practicing law, Mr. Schlefer questioned why his client, the shipping company Pacific Far East Line, was denied permission from the U.S. Maritime Commission to conduct business in the Mariana Islands, a U.S. territory in the Pacific Ocean. He was frustrated when the commission told him its reasoning was confidential.

He scheduled a meeting with the American Bar Association to suggest the drafting of a bill to gain access to government documents. He soon joined two lawyers affiliated with the ABA who had already begun working on such a bill and, together, they drafted the original version of the Freedom of Information Act, or FOIA, Mr. Schlefer recounted in a 2016 Washington Post opinion column.

After the bill won the ABA’s endorsement at its convention in Chicago, Mr. Schlefer met with Rep. John Moss (D-Calif.), who had been campaigning against government secrecy for more than a decade.

“I had planned just to leave [the draft of the bill] with him, but he asked me to sit,” Mr. Schlefer wrote in The Post. “After reading it slowly and carefully, he looked up and said, ‘Mr. Schlefer, I’ll deliver the House. You deliver the Senate.’ ”

Through a well-connected friend, Mr. Schlefer eventually persuaded Senate Judiciary Committee Chairman James Eastland (D-Miss.) to hold a hearing on FOIA legislation.

Moss remained the driving force of the FOIA legislation through Congress as it underwent revisions giving courts power to exempt certain internal agency documents related to executive privilege, national security, personnel records and criminal investigations.

President Lyndon B. Johnson signed the Freedom of Information Act into law on July 4, 1966. It has been amended several times since and remains a primary and often-used tool for making government decision-making open to public view, Mr. Schlefer wrote.

Mark Pascal Schlefer was born in Manhattan on May 9, 1922, to stockbroker parents. He graduated from Harvard University in 1943 and its law school in 1949. Between degrees, he served in the Army Air Forces in Europe during World War II. (He had been turned away from an officers’ club — despite holding the rank of lieutenant — by an unpleasant sergeant, but Mr. Schlefer appealed successfully to a serviceman of higher rank.)

Mr. Schlefer moved to Washington in 1951 and appeared to be in line for a legal position at the State Department, but the job never materialized. He learned later that his application had been flagged as a problem because his wife’s aunt, Carol Weiss King, was a prominent immigration lawyer who had represented left-wing radicals threatened with deportation.

He spent most of his career with the law firm Radner, Zito, Kominers & Fort, which later became Fort & Schlefer.

His wife of 70 years, the former Marion King, died in 2015. In addition to his daughter, of Putney, survivors include two other children, Jonathan Schlefer of Boston and Ellen Schlefer of Durham, N.H.; five grandchildren; and two great-grandchildren.

In 1963, after Mr. Schlefer grew distressed about the lack of racial diversity among students at his children’s private schools, he co-founded what is now the Black Student Fund to provide tuition assistance for minority families at Washington-area private schools.

He also served as board chairman of the Lawyers Alliance for World Security, which lobbies for nuclear arms control. He moved to Putney from Washington in 2005 and retired from law practice two years later.

His most enduring contribution to the law remained the FOIA legislation.

“Those of us who drafted the legislation and worked to obtain its enactment never expected the statute to have the long-term importance that it has achieved,” Mr. Schlefer wrote in his self-published biography, “Incidents in a Life: The War is Over, I’d Like a Glass of Champagne.” “We had a local problem, and we sought to fix it. In fact, we ended by fixing a crucial national problem.”

Russia’s Foreign Intelligence Service, known as SVR, announced his death on Dec. 26 but provided no further details. Russian President Vladimir Putin praised Mr. Blake as a “brilliant professional” and a man of “remarkable courage.”

News accounts from the 1960s described Mr. Blake as a “Super Spy,” and perhaps one secret to his successful treachery was that he hid in plain sight. As one of his friends, a Salvation Army executive, told a reporter at the time, Mr. Blake resembled “a typically blasé bowler-hatted, rolled umbrella government official.

In fact, he was the last high-profile survivor of a string of British turncoats who spied for the Soviet Union during the 1950s and 1960s, a badge of dishonor that included the Cambridge Four: Anthony Blunt, Guy Burgess, Donald Maclean and Kim Philby.

Dick White, a former chief of British intelligence, once said Mr. Blake wrought the most damage. The information he turned over reputedly led to the deaths of scores of highly placed Western agents, including Robert Bialek, a top-ranking East German police official.

He also betrayed to his Soviet handlers a joint mission between British and U.S. intelligence known as Operation Gold. The goal was to dig a tunnel underneath East Berlin to tap Soviet phone lines in the early 1950s. Mr. Blake sabotaged the multimillion-dollar operation before a shovel had ever struck German soil.

“There was not an official document on any matter to which I had access which was not passed on to my Soviet contact,” Mr. Blake confessed at his closed-door trial in 1961, news accounts reported at the time. According to a CIA report, Mr. Blake passed more than 4,720 pages of classified documents to the Soviets.

Mr. Blake spent nearly a decade leading a double life before he was arrested, tried and sentenced to 42 years in prison for espionage. At his trial, the presiding judge, Lord Chief Justice Hubert Parker, said that Mr. Blake had “rendered much of [Britain’s intelligence] best efforts useless.”

Five years into his term, Mr. Blake escaped in the middle of the night using a ladder made of knitting needles and rope. He was smuggled into East Berlin while hidden inside a secret compartment of a camper van and later traveled to the Soviet Union.

In his adopted motherland, Mr. Blake was bestowed with the Order of Lenin, the highest civilian award in the Soviet Union. A countryside dacha, a Volga car and a pension, along with his ribbons for courage and dedication to the communist cause, were the trappings Mr. Blake earned for his 9½ years of service to the KGB.

“It is hard to overrate the importance of the information received through Blake,” Sergei Ivanov, an official for the Russian Foreign Intelligence Service, told Russian media in 2007. “It is thanks to Blake that the Soviet Union avoided very serious military and political damage which the United States and Great Britain could have inflicted on it.”

He was born Georg Behar on Nov. 11, 1922, in Rotterdam. His mother was Dutch, and his father was a Jewish businessman of Middle Eastern descent who earned British citizenship while fighting for the Allies in World War I. After his father died, his mother married a man with the last name Blake.

As a youth, he spent summers with family in Cairo. It was there, Mr. Blake later said, that his interest in communism was sparked by cousins with left-wing views.

During World War II, Mr. Blake served in the Dutch underground as a bicycle courier before making his way to Britain. He joined the British navy in the early 1940s and, because of his facility with languages, was recruited to British intelligence.

As a covert officer for the British Secret Intelligence Service — often called MI6 — Mr. Blake held posts in Vienna, Berlin, Milan and Beirut. He studied Russian at Cambridge and developed a specialty in the Soviet Union.

He was assigned to the British diplomatic mission in Seoul when North Korean forces invaded the capital city in 1950. Mr. Blake spent 34 months as a prisoner, subsisting on a diet of rice and turnips in a North Korean camp. With the other prisoners, Mr. Blake sometimes displayed his creative ability to take on personas.

“He loved to imagine himself . . . a great officer of the crown ennobled for gallant and devoted service,” journalist Philip Deane, who spent time imprisoned with him in North Korea, wrote in The Washington Post in 1961. “Lightly we would tap him on the shoulder and say solemnly: ‘Arise, Sir George.’ We promoted him to baron, earl, marquess. He never quite made duke; captivity ended too soon.”

Mr. Blake said his decision to spy for the Soviet Union came after witnessing what he described as atrocities perpetrated by the West. In the PBS documentary “Red Files,” Mr. Blake described watching American bombers obliterate small Korean villages.

“It made me feel ashamed of belonging to these overpowering technical superior countries fighting against what seemed to me quite defenseless people,” Mr. Blake said in the broadcast. “I felt I was committed on the wrong side. And that’s what made me decide to change sides. I felt that it would be better for humanity if the communist system prevailed, that it would put an end to war, to wars.”

He passed a note written in Russian to his guards and was granted an audience with a KGB official, offering his services to the Soviets.

As a highly placed mole, Mr. Blake leaked secrets to the Soviets that few British spies were even cleared to know. Among the most damaging to be revealed was the Berlin tunnel operation.

Although the tunnel was built and the phone lines were tapped, no worthwhile intelligence ever resulted from the intercepts by the CIA or MI6. The projected ended up wasting the equivalent of $51 million in today’s dollars, according to the Cold War Museum in Warrenton, Va.

Instead, the Soviets used the phone lines for a disinformation campaign. To protect Mr. Blake, they allowed the operation to toil for 11 months before the tunnel was “accidentally” discovered after rainstorms washed up the handiwork of the British and American intelligence branches.

Mr. Blake’s downfall came after a Polish officer defected to the West and described — but did not identify — a top-ranking British officer who was a Soviet mole. While posted to Lebanon, Mr. Blake was called back to MI6 headquarters under false pretenses, accused and arrested.

His trial at the Old Bailey was considered so sensitive that the judge ordered the courtroom vacated, the doors locked and the windows shuttered.

Mr. Blake was found guilty, and his 42-year sentence was one of the harshest in British history for such a crime. (Klaus Fuchs, the physicist who betrayed atomic secrets to the Soviets in the 1950s, was sentenced to 14 years.)

In 2007, Mr. Blake received the Order of Friendship from Putin. “You and your colleagues made an enormous contribution to the preservation of peace, to security and to strategic parity,” Putin said in 2007. “This is not visible to the eyes of outsiders, but very important work deserves the very highest acknowledgment and respect.”

No information on survivors was immediately available. His first wife, an Englishwoman with whom he had three children, divorced him after his defection. He later was said to have married a Soviet woman and had a son with her.

Mr. Blake embraced his new life with a new name: Giorgi Ivanovich Bekhter. Early on, he naively planned to drive across his new homeland in his gifted Volga.

“At the time, I knew very little about Russian roads,” Mr. Blake later said.

Instead, Mr. Blake remained in his wooded dacha outside Moscow, reading Gogol and Chekhov. He described his life in Russia as happy and peaceful. Contemplating his legacy on occasion of 90th birthday, Mr. Blake said he had no regrets.

“I do not believe in life after death,” Mr. Blake told the Rossiyskaya Gazeta, the official government newspaper, in 2012. “As soon as our brain stops receiving blood, we go, and after that there will be nothing. No punishment for the bad things you did, nor rewards for the utterly wonderful.”