Priscilla Johnson McMillan, who has died aged 92 after a fall, was the only person who could claim to have known both President John F Kennedy and his alleged assassin, Lee Harvey Oswald. As a young college graduate, Johnson was befriended by Senator Kennedy while she worked in his office; a few years later she interviewed the young Oswald soon after he showed up in Moscow wishing to defect to the Soviet Union.

After the assassination, Johnson was given exclusive access to Oswald’s Russian widow, Marina, and her ensuing book, Marina and Lee (1977), became a key document in establishing Oswald as a lone disturbed assassin. It also prompted many researchers to point to Johnson’s close ties to the US intelligence community, not least when she received similarly exclusive access to Joseph Stalin’s daughter, Svetlana Alliluyeva, when she defected to the US, and worked with her through translating her bestselling 1967 memoir Twenty Letters to a Friend.

Johnson’s career grew from her unexpected interest in Russian language and culture. Her father, Stuart Johnson, a financier, was heir to a textile fortune; he was her mother, Mary Eunice Clapp’s, second husband. Patricia was born in Glen Cove, New York, and grew up on the family’s estate, Kaintuck Farm, in Locust Valley, Long Island.

She was educated at Brearley school in New York, then at Bryn Mawr College, Pennsylvania, one of the elite “seven sisters” female colleges, where she became the first graduate majoring in Russian studies and was active in the United World Federalists (UWF), dedicated to effective world cooperation, primarily to prevent nuclear war.

After an MA at Radcliffe College (now part of Harvard University), in 1953 she joined the staff of the newly elected senator Kennedy, researching French Indochina. They became friends; he would call her regularly for chats. She denied any romance, “I didn’t love him; he was mesmerising but he was just someone I knew.” She was rejected when she applied to join the CIA, ostensibly because of her ties to the UWF. Oddly, her interviewer was Cord Meyer, who in 1947 had been the first president of the UWF; now he headed the CIA’s Operation Mockingbird, aimed at influencing media.

She did translation work for a review of the Soviet press, spending much time in Russia. On Kennedy’s recommendation, she received a grant to study the Soviet legal system, and again did translation work at the US embassy. She met Truman Capote, travelling with a US production of Porgy and Bess, and is mentioned in his book The Muses Are Heard.

In 1958 she joined the North American News Alliance (NANA), and in November 1959 arrived in Moscow just a day after Aline Mosby of United Press International had filed a story on Oswald’s defection. The US consul John McVickar, himself a CIA man, recommended she interview Oswald, who was at her hotel; her report on the four-hour session appeared in NANA-affiliated papers.

Mosby noted that Johnson lived in the Metropol, unlike other press in their state-assigned office/residence, saying “she was a very nice person and had good connections”. Johnson was one of many journalists expelled from Russia in the wake of the Russians shooting down of an American U2 spy plane; Oswald had been a radar operator at the Atsugi, Japan base from which U2s flew.

She became a visiting fellow at the Russian research centre at Harvard, returning to Russia in 1962 and writing a memorable piece for Harper’s magazine about the Soviet writer Boris Pasternak’s funeral. On her return she was interviewed by Donald Jameson, the head of the CIA’s Soviet Russia division, who described her in a memo as someone who could “be encouraged to write the articles we want … but it’s important to avoid making her think she’s being used as a propaganda tool.”

Then, in November 1963, came the news of Kennedy’s assassination by Oswald; Johnson gasped as she realised: “I know that boy.” Her 1959 profile of Oswald was immediately reprinted, but with a few changes, including a final line that did not appear in the original: “This was the stuff of which fanatics are made.”

In 1964, when Marina was being held incommunicado, under threat of deportation, Johnson moved in with her. With her Russian and knowledge of Lee, she won Marina’s trust, but her book did not appear until 1977. While researching it, Johnson co-edited a collection of essays, Khrushchev and The Arts: The Politics of Soviet Culture (1965). She returned to Kaintuck, where Alliluyeva lived while they worked on her memoir.

Johnson married the journalist George McMillan in 1966; he covered the civil rights movement in the south, and published, in 1977, Making of an Assassin, showing how Martin Luther King’s alleged assassin, James Earl Ray, acted alone. They divorced in 1980.

Marina and Lee: The Tormented Love and Fatal Obsession Behind Lee Harvey Oswald’s Assassination of John F Kennedy finally appeared, coincidentally, just as the House select committee on assassinations reopened the case. Johnson testified in closed session; large sections of her HSCA testimony are redacted whenever she is asked about her intelligence connections. Her book was a major influence on Norman Mailer’s Oswald’s Tale; Mailer blamed Oswald’s killing of the president on his sexual frustration with Marina, and jealousy of JFK. By this time Marina began to distance herself from Johnson’s conclusions, saying “it was up to Priscilla to fish out all the facts and everything”.

In 1988, Johnson added another line to her Oswald interview, telling Dan Rather of CBS that Oswald had told her: “I want to give the people of the US something to think about.” Eventually, Marina would claim she was “misled by the ‘evidence’ presented to me by government authorities … I am now convinced Lee was an FBI informant and did not kill President Kennedy”.

Priscilla’s obituary of Edward Teller, father of the H-bomb, in the Bulletin of Atomic Scientists led to her being asked to write about the hearings that declared J Robert Oppenheimer, the “father” of the atomic bomb, a security risk when he opposed building an H-bomb.

She received extensive access to the archives of the Los Alamos Atomic Laboratory, but as with Marina and Lee, the research overwhelmed the writing. When The Ruin of J Robert Oppenheimer and the Birth of the Modern Arms Race finally appeared in 2005, it was a year after a massive Oppenheimer biography by Kai Bird and Martin Sherwin had won a Pulitzer prize. But her portrayal of the political shift that left Oppenheimer on the outside won praise.

On the 50th anniversary of the assassination, Marina and Lee was reissued. Johnson wrote of Oswald’s “unfitness for any conspiracy outside his own head”. Oddly enough, the description also would suit a hapless someone who was, as Oswald himself claimed, a “patsy”.

Johnson is survived by a niece, Holly-Katharine Johnson, who is working on her biography.