FOR Mimi Alford the first question is "why?" Why now, almost 50 years after John F Kennedy's death, would the one-time intern emerge from the shadows to claim she had had an affair with the former president while he was in the White House?

Surely every detail of JFK's admittedly colourful sex life, has, at this point, been exhausted of scandal (his legacy won't be affected; the world forgives him every dalliance). And for an apparently well-off divorcee with a comfortable life on Manhattan's salubrious Upper East Side, it hardly seems an obvious move to recast yourself as the Monica Lewinsky of your era.

For the still beautiful 68-year-old the answer is simple: this was thrust upon her; it was either have others tell her story or tell it herself. For 41 years she had lived in "fear" of being found out. She had raised two children and divorced her husband and was quietly congratulating herself on making it that far. Then, one apparently innocuous day nine years ago, she sat down with her morning coffee and opened her email to see a link that a friend had sent her about a cover story in the New York Daily News, with the headline "Fun and games with Mimi in the White House". He had sent it to her because of the "funny coincidence" of her name matching the subject of the story. She felt as though "the wind had been knocked out of her". For years, the cottage industry of JFK biographies had whispered about his affair with a White House intern but this was the closest anyone had come to "outing" her. She decided to wait out the storm, but when Celeste Katz, a reporter with the Daily News, rang her and bluntly asked if she was the Mimi of the story, Alford says she saw no point in denying it -- they would find out anyway, she reasoned. From that day forward she was deluged with offers to tell her story. The major television networks clamoured for her signature, the National Inquirer put an envelope stuffed full of money under her door -- she donated it to the church. She turned down all offers, emerging from her apartment only to hand out a written statement to the awaiting army of reporters. It said: "From June 1962 to September 1963 I was involved in a relationship with President Kennedy. For the past 41 years it is a subject I have not discussed. In view of the recent media coverage I have discussed the relationship with my children and my family and they are completely supportive." And that, as they say, was all she wrote.

Until now.

In a sensational memoir entitled Once Upon A Secret, Alford gives an account of the affair. She describes herself as fulfilling an almost concubine-like function, summoned by one of Kennedy's aides when the president needed to "unwind" after a long day. The sexual detail in the book is easily as graphic as that in Lewinsky's but one detail, repeated endlessly on American news networks, veers from the tawdry to the pitiful: the president never kissed her on the lips.

"At the time I don't think I understood the significance of that," the still-beautiful Alford told me in an interview from her home in New York. "I didn't feel used. I felt, in a way, special. I liked being part of it. I'm not sure I even understood then what that means; that them not kissing you means that there is a level of intimacy they don't want to go to. Looking back now, I can see that that was a very sad aspect of the relationship."

That detail must have seemed all the more unbecoming because Marion Beardsley, as she was born, was a good girl from a good family. She was "as cloistered as you get could without taking vows". Her father worked in trusts in New Jersey while her mother, with the "silky charms" of a society hostess, raised a family in the WASP-y idyll of their period mansion. Mimi, as she became, attended the same boarding school -- Miss Porter's in Connecticut -- that Jackie O had gone to, and had, in fact, leveraged the connection with the first lady to win herself an internship at the White House. Mildly anorexic in her teens, the statuesque Mimi nevertheless cut a stunning figure as she boarded the train to Washington on that muggy first afternoon in Trenton, New Jersey, in June 1962. She was wearing her best dress and hoped it didn't wrinkle too much in the humidity. So innocent was she that when she was first offered a tour of the White House, her first thought was of the doll's house she used to play with. She was introduced to the other women, including two, dubbed Fiddle and Faddle, friends who would themselves later be described as JFK mistresses and whose job it was to show the new girl the ropes. Ostensibly this came in the form of demonstrating how wires came into the White House press office and how photos were to be filed. Mainly, however, it came in acting as if nothing unusual had happened when the president's senior aide invited the boarding-school beauty for a cocktail and a late-afternoon swim at the White House pool. Kennedy had a bad back, for which afternoon swims with gorgeous girls were apparently the only relief. The swims, and the girls, were needed daily. Swimsuits in all sizes hung on hooks by the White House pool. A woman who had forgotten hers would have no excuse. And just at the opportune moment the other staff knew when to melt away. The way Alford describes it, the president almost forced himself upon her, with a nonchalant persistence that caused some to whom she told the story to mention the "R" word.

Despite the great disparity in power today Alford says she does not believe she was raped. "It's been a difficult question for me to come to an understanding of, but I never felt I was raped. Because I remember being willing. I don't remember being coerced. I can only say what my own feelings were."

Afterwards she went back to the house she shared with another of the White House "girls". She took a long look at herself in the mirror. She was no longer a virgin. Her first time had been with the most powerful man in the world -- his cologne still clung to her. As tempting as it was, she knew she wouldn't be telling her girlfriends about this.

"1962 was a different era," Alford says today. "I didn't discuss matters like these with anyone -- not my girlfriends or my mother. I don't think I was that unusual in this. Now in hindsight however I can see the effect of that. It disconnects you from people. They don't know you completely. One of my greatest regrets is not telling my mother -- not necessarily in 1962, but later in life, in the Seventies and Eighties. She died in 1998. I didn't process it for myself either because I kept it a secret."

For the rest of the summer of 1962, Alford says, she and the president turned his private West Wing bathroom into their own private spa. He taught her how to cook eggs the way he liked them. They listened to music together -- she playing him the teen classics of the day and he attempting to convert her to Tony Bennett. When she left, the journey home never felt like a walk of shame. "I felt invulnerable, as if I were cloaked in the president's power," she writes.

Much has been written about JFK's sex life and one of the questions that people come back to is how the press didn't find out. Mimi's explanation is that their world was more Mad Men than West Wing -- people did know about marital indiscretions, but they turned a blind eye. She cites an anecdote told by former White House press secretary Pierre Salinger, who described how easily he swatted away a query about the president's dalliances. "I gave him [the reporter] a Sixties answer, not a Nineties' answer. 'Look, he's president of the United States. He's got to work 14-16 hours a day. He's got to run foreign and domestic policy. If he's got time for mistresses after all that, what difference does it make?' The reporter laughed and walked out. That was the end of the story."

As inchoate as she was, Mimi sensed there had been other women before her. There had to have been -- the whole "seduction" was as smoothly coordinated as a car assembly line. She calls JFK "The Great Compartmentaliser" -- a man adept at keeping the various worlds he moved in entirely separate. One of these worlds was in the White House's East Wing, where the first lady, the woman whom Mimi once wrote to requesting an interview for the school paper, had her office. At the time, Alford writes, she felt not a twinge of pity for Jackie O. Today, however, she feels a tincture of remorse: "It's very difficult for me to admit that as a 19-year-old I didn't feel guilty. I was intruding on her marriage but at the time I didn't feel that way. It could partly have been the power of the president -- taking cues from him. She [Jackie] had a lot of tragedy in her life. I feel sad about that. I do have guilty feelings about what I did. But my book is not really about Mrs Kennedy -- it's about me."

At the end of the summer she went to college but Kennedy kept in touch, calling her using the pseudonym "Mr Carter" (sounding like "Cotta" in his Massachusetts drawl) and she regaled him with teenager stories. Every so often, she would be spirited back to Washington, her flight mysteriously paid for, her accommodation taken care of. Nobody asked her any questions -- it was assumed she was staying with a friend. Occasionally, the family might have commented on her tan or the press might have wondered who the leggy girl hiding under the dashboard in one of the presidential cars was, but nobody made a fuss. She was continually invited to take official trips with JFK and always she waited for him in his hotel room.

Alford says the first time she saw Kennedy in political or historic, rather than personal, terms was during some key events during the civil rights movement (a black student had gone to court to be allowed go to university in Mississippi) and, even more memorably, during the Cuban Missile Crisis. She remembers the other girls in the school watching the drama unfold on television. When she got to Washington the president was tired and drawn, telling her he'd "rather see his children red [Communist] than dead". As the tension reached its climax, people began fleeing Washington for Florida. Mimi, meanwhile, was "sleeping like a baby, wrapped in soft linens, in a bedroom on the second floor of the White House. At that moment it felt like the safest place I could be".

At other times, she felt less safe, however. At a party at Bing Crosby's house she once took drugs (she's not sure which ones, her recollection seems hazy) and felt palpitations. Kennedy spent the night partying in his own wing of the house while she slept on her own. In another episode, which she calls "callous", she says the president told her that Dave Powers, special assistant to the president, looked a little "tense" and would she "take care of it". She understood this as a challenge to give Powers oral sex and was appalled. Then she complied. It was, she says now, a "sordid" scene, and Powers was "jolly and obedient" as she "performed my duties".

Discussing matters like this was difficult, she now says, but she tried to find a balance between the discretion she was raised with and today's culture of confession. "The pressure of the last few days has been intense. Keeping the secret for so long had a lot to do with my voice developing as a person. Of course some things were extremely difficult to write about. But I think you have to find a balance. I hope I've done that."

Beyond the rapacious philanderer there was a softer side to Kennedy. Alford tenderly recalls his all-encompassing grief at the death of his son, Patrick Bouvier Kennedy, who was born premature, with respiratory stress syndrome and died. She vividly recounts the president sobbing and bereft as he answered letters of condolence.

In time, she found a boyfriend her own age, a college student named Tony Fahnestock, and found that an unexpected effect of her liaison with the president was that she was more eager for sex than the young man she was dating. She also felt that she loved him but that the affair with JFK had inhibited her from the thrill of falling in love. It was, she says, one of the many ways her secret cost her. She was eventually engaged to be married to Fahnestock -- the engagement notice ran in the New York Times. She still went on trips with JFK but when he blithely asked her to perform the same act on Teddy as she had done on Powers, she was now self-confident enough to refuse. The president's "wedding present" to her came in the form of $300 shopping money given to her at the Carlyle Hotel in New York. It was also the last time she would ever see JFK. He told her he wished she were going with him to Dallas. She reminded him she was getting married and hopped in a car, laden down with shopping bags.

The day he was shot, people walked the streets, "with their hands to their mouths in stifled sobs". At the home of her staunchly Republican in-laws-to-be they opened the afternoon cocktail bar a little early and handed her a glass of whiskey. It was only later, while truly distraught, that she confessed to her fiance. He responded by giving her the silent treatment and then having sex with her -- the first time they had done so. In what was becoming a recurrent theme, she meekly complied. When Kennedy's funeral was broadcast she stared impassively at the television screen, a shell of the bright young thing she had once been. The marriage went ahead, and a year after the assassination she became pregnant but the child died a day after he was born -- of the same respiratory ailment that killed the infant Patrick Kennedy. In 1968 she gave birth to a daughter, Lisa, then another, Jenny, and she and her husband, who had found a job at Goldman Sachs, settled into the comfortable New York City lifestyle that seemed their birthright. The marriage eventually hit the rocks, however, and they divorced.

Over the years she destroyed the mementos Kennedy gave her and shared her secret with only a handful of confidantes. It would be the revelations of the Daily News that would finally push her over the edge. She told her daughters, who were supportive, but it was the supportive letter of a stranger -- Dick Alford -- that would prove most fateful. When Mimi first received the letter, she put it in a file and thought no more of it. Over the following few months however she would go on dates with various men, and find herself dispirited by the process. The writer of the letter came back into her mind -- he suddenly seemed more intriguing -- and she retrieved the file for his contact details. They met and discovered that they had friends and interests in common. He would eventually become her second husband. (Today she calls him, "one of the great things to come out of the whole experience".)

In 2009 they visited JFK's grave at the Arlington National Cemetery in Virginia together. It was the final chapter in a story that would soon be told.

Alford says she doesn't even consider herself worthy of a historical footnote but she has found a degree of peace with the decision to tell her story.

Her own family, she insists, will not be hurt by the revelations. The Kennedys themselves, however, are another story. They have long railed against the depiction of JFK as the philanderer-in-chief. Alford says that she hasn't heard from them and their possible reactions did not have a bearing on her.

"You have to understand I wrote this book for myself -- it's a memoir -- and it goes beyond the eighteen months of my time with Kennedy ... . The importance was the impact of keeping a secret in someone's life.

"This opens a conversation for other people who've kept secrets, whatever they are. I focused on myself ... this was about me. I can accept other people's judgments and opinions. That's why I feel so liberated today."

Once Upon A Secret is published by Hutchinson, priced £12.99

- Donal Lynch