Just out of graduate school in 1953, Priscilla Johnson McMillan joined the Senate staff of John F. Kennedy, then a newly elected Democrat from Massachusetts. He was “mesmerizing,” she later said; while she worked only briefly on Capitol Hill, she visited him in the hospital when he underwent spinal surgeries, and posed as one of his sisters to get past a line of nurses and bring newspapers to his bedside.

Mrs. McMillan, who was then known as Priscilla Johnson, later went into journalism and moved to Moscow, where she drew on her fluency in Russian to file stories for the North American Newspaper Alliance. In November 1959, a friend at the U.S. Embassy mentioned that “a boy named Oswald” was in town trying to defect. He was staying at her hotel, the Metropol, where she spent five hours interviewing him over tea.

The young man seemed excited, nervous, a little frightened. He was 20, a former Marine with a light Southern accent, and wanted to talk about Marxist economics and complain about the U.S. Embassy, which he said had tried to dissuade him from renouncing his citizenship. “I want to give people in the United States something to think about,” he said.

Four years later, on Nov. 22, 1963, Mrs. McMillan was suddenly jolted back to their conversation, not long after learning that Kennedy, as president, had been assassinated in Dallas. Walking through Harvard Square, near the university where she was a visiting scholar, a friend told her that authorities had arrested the shooter. His name was Lee Harvey Oswald.

“My God,” Mrs. McMillan recalled saying. “I know that boy.”

Indeed, she was one of the only people who knew both Kennedy and his killer, who died two days later after being shot by nightclub owner Jack Ruby in the basement of Dallas police headquarters. Their deaths launched her on a 14-year odyssey, as she tried to find out why the quiet young man she met in Moscow had decided to shoot the president.

Mrs. McMillan persuaded Oswald’s Soviet-born widow, Marina, to sit for an exclusive book interview in exchange for a share of the royalties. They wound up speaking for nearly seven months, providing Mrs. McMillan with the core of “Marina and Lee” (1977), a critically acclaimed account of the Kennedy assassination, told through the lens of Oswald and his wife.

In a review for the New York Times, Pulitzer Prize-winning journalist Thomas Powers wrote that Mrs. McMillan’s book “achieves with art what the Warren Commission failed to do with its report,” offering a persuasive case that Oswald acted alone as the assassin.

“It is far better than any book about Kennedy,” he added, “with the unsettling result that the assassination is experienced from the wrong end. . . . If you can find the heart to read it, you may finally begin to forget the phantom gunmen on the grassy knoll.”

Mrs. McMillan, who went on to an accomplished career as a historian of the Cold War and U.S. nuclear weapons policy, was 92 when she died July 7 at her home in Cambridge, Mass. Her health had declined after a fall about eight weeks ago, said her niece and biographer, Holly-Katharine Johnson.

While writing her Oswald book, Mrs. McMillan translated “Twenty Letters to a Friend,” a 1967 memoir by Stalin’s daughter, Svetlana Alliluyeva, who had defected to the United States earlier that year. She later spent more than two decades researching and writing “The Ruin of J. Robert Oppenheimer” (2005), about the father of the atomic bomb, whose career unraveled after he was accused of being a Soviet spy during the McCarthy era.

But she remained best known for her book on Oswald. His widow, who remarried and went by Marina Oswald Porter, described him as a fame-obsessed liar with a short temper and violent mood swings. “He was a lonely person,” she told Mrs. McMillan. “He trusted no one. He was too sick. It was the fantasy of a sick person, to get attention only for himself.”

By the time Mrs. McMillan published her book, conspiracy theories had proliferated about the killing. There seemed to be little appetite for her relatively straightforward account of a wayward, self-described Marxist; sales were modest, although “Marina and Lee” was reissued in 2013.

“The argument over Kennedy was a kind of national madness for decades — but that is largely over now, and I would argue that Priscilla’s book stands firm as balanced and persuasive,” Powers wrote in an email. Mrs. McMillan’s interviews with Marina and Lee Harvey Oswald, he added, formed a key part of the historical record.

“Imagine that some Roman had done the same with Brutus before the assassination of Julius Caesar, and then followed it with a similar history of the countdown to the killing — if you wanted to understand the politics and the life of Rome in those years, that is where you would start.”

Priscilla Mary Post Johnson was born in Glen Cove, N.Y., on July 19, 1928, and raised in nearby Locust Valley, on the North Shore of Long Island. Her father was a financier who inherited a textile company, and her mother was a homemaker.

After graduating from the private Brearley School in Manhattan, she studied Russian at Bryn Mawr College, receiving a bachelor’s degree in 1950. Three years later, she earned a master’s in Russian studies from Radcliffe College, now part of Harvard.

Mrs. McMillan translated Russian newspaper articles before traveling to the Soviet Union for the first time, in 1955, paying her way by working as a translator for the New York Times. In Leningrad, now St. Petersburg, she palled around with newspaper columnist Leonard Lyons and novelist Truman Capote, who recounted some of their experiences in a 1956 nonfiction book, “The Muses Are Heard.”

In 1966, she married George McMillan, an author and journalism instructor. They later divorced. She had no immediate survivors but had a vast “chosen family,” often letting near-strangers and mutual friends stay at her home in Cambridge, where she was an associate at Harvard’s Davis Center for Russian and Eurasian Studies.

“More than anyone I’ve ever met, she created something like a 19th-century European salon at her home,” said Steven Aftergood, the director of the Government Secrecy Project at the Federation of American Scientists. “You’d never know who you’d meet — government officials, academics, writers, artists. It was a kind of intellectual chemistry experiment.”

In recent years, Marina Oswald insisted that her husband was actually innocent, and blamed the Mafia and CIA for Kennedy’s killing. Mrs. McMillan remained convinced that Oswald acted alone, telling the Atlantic that “Marina’s change of views may stem from her daughters’ reluctance to accept their father as the assassin.”

She had long believed that the assassination would prompt conspiracy theories, in part for psychological reasons. “The killing of a President, or a king or father, is the hardest of all crimes for men to deal with,” she wrote in a 1975 Washington Post essay. “As Freud pointed out, it is this crime that stirs the deepest guilt and anxiety. . . . No matter what steps are taken, what investigation may be authorized or what autopsy material made public, I suspect that the doubts about President Kennedy’s murder are going to be with us forever.”