Plato Cacheris, for decades one of Washington’s premier defense lawyers, with prominent legal roles in the Watergate and Iran-contra scandals and President Bill Clinton’s affair with White House intern Monica S. Lewinsky, died Sept. 26 at a nursing home in Alexandria, Va. He was 90.

The cause was pneumonia, said his wife, Ethel Cacheris.

Mr. Cacheris, the son of Greek immigrants, belonged to a select coterie of Washington lawyers who created their own peculiar legal specialty, made it highly respectable and went on to dominate it for decades: a white-collar defense bar for those ensnared in the capital’s major political cases.

For much of the early 20th century, prestigious law firms generally regarded criminal defense work as undesirable, even slightly tawdry — the domain, as Mr. Cacheris once put it, “of rather slick characters.”

The members of his small fraternity — and in those early days, they were nearly all men — represented the principal figures in the major dramas that increasingly played out when law and politics intersected in a courtroom.

In their stolid public manner and bespoke dark suits, Mr. Cacheris and his counterparts took as their model Edward Bennett Williams, the nationally known Washington lawyer who showed that representing high-profile criminal defendants could bring cachet and prosperity.

In the Watergate break-in and coverup, Mr. Cacheris represented former attorney general John N. Mitchell; in Abscam, which involved federal agents masquerading as wealthy oil sheikhs offering bribes to members of Congress, he represented then-Rep. Michael “Ozzie” Myers (D-Pa.); in the international scandal involving the rogue bank B.C.C.I., he represented Sheik Kamal Adham, the former Saudi intelligence chief; and in Iran-contra, he was the lawyer for Fawn Hall, a secretary to Lt. Col. Oliver L. North at the National Security Council.

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Mr. Cacheris also formed a high-profile subspecialty in espionage cases. His clients included Aldrich H. Ames of the CIA and Robert P. Hanssen of the FBI, both of whom spied for the Soviets and then the Russians, and Ana Belén Montes of the Defense Intelligence Agency, who spied for Cuba.

Prosecutors often were pleased to face Mr. Cacheris — not because he was anything less than a formidable adversary, but because he was experienced and deft in the ways of high-stakes legal contests, from candid plea negotiations to, if necessary, showdowns in the courtroom.

“You could always rely on whatever he told you,” recalled Chuck Rosenberg, a former federal prosecutor in Virginia. “Moreover, his life showed attorneys that one can be civil and decent and also greatly effective.”

His skill at dealing constructively with prosecutors was on vivid display when he represented Lewinsky, who initially denied to law-enforcement authorities her dalliance in the Oval Office with Clinton. She found herself facing felony charges as a target of independent counsel Kenneth W. Starr.

Lewinsky was first represented by William H. Ginsburg, a medical-malpractice lawyer from California and family friend. Ginsburg, who had little experience in either Washington or criminal law, made a marathon of media appearances and proved spectacularly unsuccessful in helping Lewinsky’s cause. Prosecutors refused to deal with him after he publicly called Starr “a monster.”

The Lewinsky family jettisoned him in June 1998 and hired Mr. Cacheris and his longtime friend Jacob A. Stein, another pillar of the elite D.C. defense bar. In short order, they obtained for Lewinsky complete immunity from prosecution in exchange for testifying truthfully and completely.

Although critics tried to depict Mr. Cacheris and others in his field as accomplished mostly at dealmaking, he ably demonstrated his trial skills when necessary.

In 1985, Mr. Cacheris successfully defended David K. Davoudlarian in a civil case brought by his stepdaughters, who said that the Virginia gynecologist had strangled his wife in their Annandale home two years earlier.

“My client is insensitive, a fool, an ass, a boor,” Mr. Cacheris told the jury. “But he is not a coldblooded strangler.” The case ended with a hung jury and was settled out of court.

Two years later, Mr. Cacheris received a tremendous burst of public attention representing Hall, the 27-year-old secretary who had been implicated in the Iran-contra scandal for shredding and removing documents from the White House. Her boss was North, the key operative in the Reagan administration’s illegal plan to secretly sell arms to Iran and use the profits to help right-wing Nicaraguan rebels known as the contras.

Hall eventually received immunity in exchange for testifying before the Iran-contra committee on Capitol Hill and at North’s trial. In 1989, North was convicted of destroying government documents and other felonies but later won a reversal on the grounds of having been given immunity for his congressional testimony.

Reflecting on his roster of clients, Mr. Cacheris said he defended the person, not the crime. “I try to see that their rights are protected,” he told the South China Morning Post in 2014. “I don’t lecture them on what they’ve done; I don’t believe in that. What’s done is done; now the question is ‘how can we come out of it the best way possible?’ ”

Plato Chris Cacheris was born in Pittsburgh on May 22, 1929, and spent his teenage years in the Washington area, where his father helped run a chain of hamburger and waffle shops.

He graduated in 1947 from the old Western High School in the District and in 1951 from Georgetown University. During the Korean War, he enlisted in the Marine Corps officer candidate school program and served as defense counsel in special courts-martial cases. On the G.I. Bill, he graduated from Georgetown’s law school in 1956 and was particularly inspired by a course taught by Edward Bennett Williams.

In 1955, he married Ethel Dominick. In addition to his wife, of Alexandria, survivors include two children, Lisa Burnett of Arlington, Va., and Byron Cacheris of Alexandria; a brother, James Cacheris, who became a federal judge; and a granddaughter.

Mr. Cacheris formed a private criminal defense practice in 1965 after several years working as a Justice Department prosecutor and an assistant U.S. attorney in Alexandria. “I like defending better, although it’s harder,” Mr. Cacheris told The Washington Post. “I like the battle against the government. And I never really liked seeing people go to jail.”

Courts assigned him to represent two of his most notorious clients, Ames and Hanssen.

Ames, the most notorious turncoat in CIA history, was arrested in 1994. He had been spying for the Russians since 1985 and had given up the names of at least 10 agents who were supplying information to the United States.

Ames agreed to plead guilty to espionage charges and to fully reveal his activities in exchange for a lenient sentence for his wife, Rosario, who knew about his betrayal and enjoyed the money he had received from the Russians. She served five years.

Ames did not face the death penalty. But because the government believed his treason led to the deaths of at least 10 Russians recalled to Moscow, the espionage law was changed to allow for execution when the government could show that an offender caused the deaths of agents working for the United States.

That was the situation faced by Hanssen, who was arrested in 2001 after having spied for Moscow on and off for 22 years, and who came to be regarded as one of the most damaging double agents in U.S. history. Mr. Cacheris argued that Hanssen had betrayed some of the same agents as had Ames, and that therefore the government could not prove their deaths were caused directly by Hanssen.

Both Ames and Hanssen — who also agreed in a plea deal to disclose his espionage work — were sentenced to life in prison.

In 2013, Mr. Cacheris secretly represented Edward J. Snowden in confidential negotiations with the Justice Department after the former National Security Agency contractor was charged with espionage and granted asylum in Russia. Snowden had provided information to The Post and other newspapers about top-secret surveillance programs operated by the NSA.

In the discussions, which came to naught, Mr. Cacheris tried to forge a bargain in which Snowden could return to the United States facing reduced charges in exchange for telling officials exactly what secrets he had disclosed and how he had done so.

Of all the cases Mr. Cacheris handled, perhaps the one that most exemplified the challenges of his trade was Watergate, the prototype of the sprawling legal and political scandals of modern-day Washington.

Mr. Cacheris and his first legal partner, William G. Hundley, represented the former attorney general who had been chairman of President Richard M. Nixon’s reelection committee. Mitchell was convicted of conspiracy, obstruction of justice and perjury for his role in the Watergate affair and served 19 months in federal prison before being released for medical reasons.

Mr. Cacheris later opined that Mitchell would have done better if he hadn’t refused to testify against Nixon, who resigned in August 1974.

The tightknit circle of established white-collar defense lawyers would often gather at the Cacheris household in Alexandria, many of them playing tennis on what was sometimes called the John Mitchell Tennis Court. Mr. Cacheris had jocularly asserted that it was paid for with the fees he got from representing Mitchell.