Late on a spring afternoon last year, my wife, Cheri, and I went to watch our eight-year-old son, Brody, play in the last baseball game of the season. Out on the field was the man who had once promised me the brightest future I could imagine and then abandoned me to national disgrace. He was hiding behind his sunglasses, talking to his son, Jack. Cheri and I sat alone, ignored, as the other parents chatted. Jack and his sister, Emma Claire, who used to play with our kids, looked at us with confusion in their eyes, and we overheard one of the mothers in the crowd whisper something about "the Youngs". When the game ended, my old friend, boss and mentor walked the long way to his car so he could avoid us. It was the last time I would see John Edwards.

It's hard to recall now, in the Obama era, but at one time Edwards was heralded as a potential saviour for the Democrats. Like many others, I'd believed he was destined to lead the party and the country. Back then, he had given me an outlet for the powerful idealism I'd first felt as a small boy who sat awestruck every Sunday as my father, a university chaplain in the turbulent 70s, had challenged prejudice and small-mindedness.

Having dropped out of university the first time – a shy young man, still coping with the scandal of my father's adultery in my late teens – I went back to get a law degree and, in 1994, became part of Democratic governor Jim Hunt's campaign. By the time I volunteered to work on Edwards's campaign, I had become a fully-fledged political junkie.

In 1998, Edwards had surprised everyone by winning the Democratic party's nomination for the US Senate. A superstar lawyer, he had amassed a personal fortune by suing on behalf of those injured by corporations, hospitals or individual defendants. His most famous case involved a little girl who barely survived after being disembowelled by the suction of a pool pump. At the end of the trial, Edwards gave a 90-minute closing speech in which he evoked the recent death of his teenage son, Wade, in a car accident, in 1996. His performance won a $25m verdict for his clients.

At 45, Edwards looked as if he was in his mid-30s and brimmed with confidence. Having grown up the son of a mill worker in North Carolina, he spoke about healthcare and education with personal authority. During high school, he had worked cleaning soot off ceilings in the mill. He'd been surprised when he got into law school and surprised again when the most worldly, sophisticated and beautiful woman in his class, Elizabeth Anania, agreed to marry him. She, too, had a successful legal career, but after Wade was killed, she retired and underwent fertility treatment, giving birth, at 48, to Emma Claire, then later Jack.

Washington seemed to love John Edwards, and I worked overtime for him almost every day. I quickly became the senator's "go-to guy" and, as she got to know me as a reliable aide, Mrs Edwards couldn't have been friendlier. If it was late and she was in bed, she told me just to come on into the bedroom and put the suitcases in the closet. "It's all right, Andrew," she would say. "You're family."

Serious talk about Edwards running for national office began in June 2000 and, at a time when Bill Clinton's extramarital affairs had stirred public outrage, John and Elizabeth presented themselves as a blissfully married couple. You could see she loved him and respected him – but his candidacy, like his career, was a joint project, and he invited her criticisms. She edited every statement he issued, no matter how minor.

In 2004, John Kerry got the Democratic presidential nomination over Edwards, and while Kerry deliberated over who to ask to join his ticket, Edwards invited my family to join his at Disney World, paying for the trip. In retrospect, I'd guess we mounted this expedition primarily so the world would know the Edwardses were at the most American vacation spot if the call came for him to be vice-president, but it was a thrill for the kids. In fact, it wasn't until we were back home that Kerry finally phoned Edwards to ask him to be on the ticket. The senator called me and said, "We're in, Andrew, and we're going to win it."

We didn't win. And by the time the votes were being counted, Edwards had other concerns on his mind: his wife had been diagnosed with cancer. This news appeared in the media at the same time as it was filled with analysis of the election and articles predicting the senator's political future. Though he was widely regarded as the Democrat most likely to challenge Hillary Clinton for the 2008 presidential nomination, he downplayed his prospects, noting that for the foreseeable future he would be focused on his wife's health.

Her cancer was aggressive, and the gruelling treatment took six months. The senator divided his attention between her, their children and his own struggle to set a course for his political future. During a week when Elizabeth was between treatments and strong enough to be alone, the senator and I went away together, to rest and brainstorm.

While we were away, the senator told me his wife was certain he should mount another campaign, whether or not she was sick. He complained about her more than usual. She had been picking fights, often about staffers she didn't trust (she didn't trust most of his aides). "Sometimes I think she's crazy," he said. But it was clear that, despite the obstacles he might face, he believed he'd occupy the Oval Office in January 2009. And I'd continue to work for him as a sort of personal assistant.

When we got back, Mrs Edwards began planning the construction of her dream house. My relationship with the Edwardses had grown so close that they felt comfortable asking me to do anything. So I became informal project manager, in charge of chasing down the architect and contractors.

I believed I would work for the Edwardses for many years to come, so felt confident taking on the expense of building a new home of our own nearby. But my wife Cheri worried about how entangled my life (and hers) had become with theirs. I didn't listen when she said that I had come to identify too closely with the Edwardses, at the expense of my own priorities and hopes.

Instead, I trained myself to ignore the senator's self-centredness and overlooked incidents where he seemed hypocritical. When I heard about him having extramarital affairs, I refused to believe the stories. When he arranged for the campaign to pay $100,000 to a woman with almost no experience for a video project, this mysterious person, Rielle Hunter – a woman the senator had met in a bar – was put on the payroll.

The senator first met her in early 2006. Born Lisa Druck and raised in Florida, she had dropped out of college and moved to New York City in her early 20s, earning a reputation as a sexually liberated party girl. In 1991 she married attorney Alexander Hunter and changed her name to Rielle Hunter. She did a little acting, produced a short film and studied new age religion. By the time she met Edwards, she was 41, divorced, unemployed and living rent-free with a friend.

According to Rielle, when she first saw him, he gave off an "energy" that told her he could be a powerful force for peace and progress, like Martin Luther King Jr. She believed their souls had known each other in a previous life, and decided she'd devote herself to helping him reach this potential.

The senator introduced Rielle to me as a filmmaker who was going to make brief documentaries – "webisodes" – to air on the internet, but I soon had an idea of what was going on between them. Rielle worried me. She was flashy and loud, and I was concerned she might do something to make public her relationship with the senator.

After one trip, when the senator stumbled into the house exhausted and flopped into bed, a secret cell phone he had left in his suitcase began to ring. Mrs Edwards answered. Without hearing a "Hello", Rielle launched into a romantic monologue about how much she missed the man who was supposed to answer. After ending the call, Mrs Edwards went to the senator and demanded to know what was going on. He confessed to having had a one-night stand and she accepted this explanation, but demanded he return the call and, as she watched, end the relationship.

He did as he was told, but as soon as he was able he telephoned Rielle again to reassure her they were still in a relationship. The senator told me Mrs Edwards was being overly demanding, obsessive, even "crazy". But he also said that he would never seek a divorce. For one thing, he still loved Elizabeth. And he believed that his wife was more popular with many voters than he was, and that if he left her, he might as well forget ever becoming president. I cringed when he said this, but when I thought about how it would look if he divorced a wife of almost 30 years, who had lost a child in a car crash and was living with cancer, I had to agree with him about the political impact.

Elizabeth wrote a book about her battle with cancer and, while she was on her book tour, the senator brought Rielle to his home, where she met Jack and Emma Claire, and even interviewed them briefly on film. When I arrived, the mood in the house was relaxed and upbeat. That evening Rielle talked excitedly, saying that John's future was limitless, punctuating her observations about him with a laugh and the line, "It's good to be king." As the wine flowed and the kids went to bed, the senator and Rielle dropped the pretence that they weren't involved. At one point, they started musing about how the house seemed like a happy place with Elizabeth and her "negative energy" removed. Rielle talked about living in the mansion once Mrs Edwards was out of the way.

This worried me, but I was busy with my own problems. Cheri and I had tried to celebrate our wedding anniversary, but ended up arguing about my 24-hour-a-day devotion to the Edwards family. Though I was finally earning a good salary and getting some respect in national politics, Cheri knew I was not guaranteed a long-term position. We were dependent on the Edwardses for our income and health insurance, and they hadn't shown themselves to be paragons of stability, especially since the arrival of Rielle Hunter.

Cheri was right. I was wrong. But I wasn't going to admit it. Instead, I said what I always said: "John Edwards is going to be president one day." In the heat of the moment, and having had just wine and a little bread at dinner, I drove off. I was arrested for driving under the influence and lost my licence.

I had to hire an assistant to help me get around for work, but worse was having to tell the Edwardses. Elizabeth's response confirmed all the good feelings I had ever felt for her. "Andrew, you are family," she said. "You don't worry about this… It's going to be all right."

The senator called me about half an hour later. "We've all done something like this, Andrew. I have. I know you feel like the lowest person on earth right now, but I love you. You are like a brother to me." I felt as if a great weight had lifted.

Words like "love" and "family" make you feel a powerful bond, but this can also be a trap. With the crisis around my arrest, my loyalty to John Edwards shifted from hope for a better future to dread of being exposed and losing my livelihood.

In the autumn of 2006, Mr and Mrs Edwards travelled extensively to promote themselves (she for her book, he for the White House), and the senator saw Rielle and spoke to her by phone as often as he could. Although he had not formally declared, the senator had been operating as a candidate for the 2008 Democratic party nomination ever since the 2004 defeat. He faced two main opponents in Hillary Clinton and Barack Obama.

Mrs Edwards was watching the senator closely, so Rielle bought a new cell phone we came to call "the Batphone". The senator and Rielle made it hard for me to ignore the affair – he let me see them kiss and Rielle recounted their sexual exploits to me – but he always used the lawyer's trick of speaking in code so he could claim "plausible deniability" if it was ever needed.

Similarly, Mrs Edwards chose to limit the questions she asked to delay a confrontation. The two women finally met face to face at a campaign event. Mrs Edwards glanced past Rielle, caught my eye, and quickly realised this must be "the other woman". The next day, I picked up the senator at his house and he told me Mrs Edwards had shouted and cried until he told her all about Rielle and promised to fire her. What I wouldn't find out for many months was that the senator had told Elizabeth that although he had indulged in a "one-night" fling with Rielle, in recent weeks she had become my mistress.

More than once after that Mrs Edwards called to interrogate me about Rielle. I felt he should be answering her questions, not me, but this angered her. "I know you are lying to me," she said, "and if you don't tell the truth to me, I'll have John fire you." During one call, I launched into a minor diatribe. "Mrs Edwards, I love you like a big sister, and I love your husband like a big brother," I said, "and after 10 years of me working for you, for you to treat me like this is wrong, utterly wrong."

She was not impressed. "Andrew, you are not family. You work for us. Nothing more. You get paid for all you do."

At this time, aside from managing the senator's personal crises, I dealt with inquiries from colleagues who knew I had Edwards's ear and understood him better than anyone else. I also looked after Rielle's travel arrangements, which required some fancy footwork. When I knew where the senator was staying, I made reservations in my own name, faxed copies of my credit card and state identification card, and told the hotel staff my "wife" would be checking in on my account. I used my credit card to book hotel rooms and flights for her, so Mrs Edwards wouldn't notice the money had gone. I even gave Edwards cash to give her – a few hundred dollars at a time – when I took him to the airport. He promised I would be repaid when a wealthy benefactor was recruited or when Mrs Edwards died. "I'll take care of you, Andrew," he said. "You know I'm good for it."

Elizabeth Edwards's cancer returned, and most people believed she was not long for this world. A few people criticised the Edwardses for their ambition, but in the main they were honoured for their courage and forthrightness. Frank Rich of the New York Times would even publish a column titled Elizabeth Edwards For President.

Although I continued to work as a fundraiser, Mrs Edwards had told the people in charge of the day-to-day campaign to take away my responsibilities and to keep me away from her husband and her house. Because she identified me with his infidelity, she obsessed over what I knew. She called me over and over, demanding information I wouldn't give her. She accused me of lying, cheating and even stealing from her household. In furious fights, she insisted her husband fire me, which he couldn't do because he needed me to take care of Rielle. I was squarely in the middle.

Meanwhile, Rielle and I were in constant communication. I had never known a woman who needed more attention, but on the day in May 2007 when I received four calls in an hour, I knew something unusual was going on. Having deflected her three times, I answered the fourth call feeling irritated. "Somebody better be dying or pregnant," I said.

"Nobody's dying," sobbed Rielle.

Later that day, cussing and barely under control, the senator told me that since they had an "open" relationship, he thought there was only a "one-in-three chance" he could be the baby's father. He asked me to help persuade her to get an abortion.

But Rielle refused, and every once in a while threatened to go to the press and reveal the affair. She was especially outraged on 30 July when the Edwardses marked their 30th anniversary by renewing their wedding vows. Rumours were starting to swirl, though, and when National Enquirer photographers turned up outside Rielle's house, the senator decided the temporary solution would be for her to stay with Cheri and me.

"This is bigger than any of us," he said, evoking the many causes – peace, healthcare, poverty and so on – that he represented. When I finally agreed with his plan, the senator said, "Andrew, nobody has ever done something like this for me. You are the best friend I ever had in the world."

Cheri had seen so much crazy stuff where the Edwardses were concerned that she wasn't exactly surprised. She was angry and disgusted, but she trusted me enough just to shake her head in a weary way and say OK.

But within days of Rielle's arrival, there was another tabloid story, and a few months later, when photographers caught Rielle, heavily pregnant, running errands just a few miles from the Edwards mansion, the senator called. "This is bad, Andrew," he said. "You have to get her under control." The Iowa caucuses were three weeks away. Edwards was neck and neck with Obama for the lead in the polls. Clinton was a distant third.

The next day, the senator called again. He wanted to find a "way out of this thing". He talked about how he and John Kerry had lost by a few hundred thousand votes in Ohio in 2004 and said, "A black or a woman can't win the general election." Gradually, he came round to the real purpose of his call: he wanted me to issue a statement saying I was the father of Rielle's baby, then disappear with her, Cheri and the kids for a luxury vacation until the election was over.

I was dumbfounded. How, I asked, was I meant to explain to my wife that I should confess to an affair I never had, claim an unborn child that was not mine and then bring her along with our family as we attempted to disappear? The senator appealed to my commitment to the cause and our friendship. When I said he was asking me to ruin my career and my ability to support my family, he said he would make sure I had a job in the future. "You're family. A friend like no friend I've ever had," he said, before adding that I would help make Mrs Edwards's dying days a bit easier. "I know you're mad at her, Andrew, but I love her. I can't let her die knowing this."

I didn't know whether to laugh or cry. My wife and children were caught up in this. My colleagues were shunning me, my professional contacts evaporating. And the much beloved and respected Elizabeth Edwards was telling mutual friends, donors, politicians and anyone else who would listen that I was the worst kind of scoundrel. (The senator had obviously long ago told her the lie about my being the baby's father.) In short, I was fucked, and at that moment I couldn't see any options but to continue playing Edwards's game.

Exhausted and under intense pressure, Cheri finally agreed that, even if we followed through on the senator's plan, no one who knew us would believe the story. I would work with a lawyer to craft a statement to release to the media. Then we would fly off somewhere no one could find us.

Barack Obama took Iowa, and John Edwards finished with just 1% more of the vote than Hillary Clinton. In New Hampshire, Edwards got clobbered, finishing a distant third. While some Democrats began calling for the senator to drop out, we were still hiding out with a heavily pregnant Rielle. School was starting, and Cheri and I wanted to go home. But now the senator was insisting we stay away until his part in the election was over or Mrs Edwards died.

We found a teacher who would home-school the kids, and fell into a routine of taking care of Rielle's basic needs almost as if she were our child.

The last straw for the campaign came on 26 January, when the senator finished third in the primary in South Carolina, where he was born. After this defeat, he quit the race, but even then it was impossible to get him to focus on resolving our situation. As the days passed and the birth of Rielle's baby drew closer, Cheri and I became less confident about the promises he had made.

On Wednesday 27 February 2008, Rielle's baby girl was born. I called and texted the senator, and he gave me a vague assurance that he would call her. I asked if he wanted me to send her flowers from him. "Yeah, that's a good idea." He paused. "But don't sign it from me. Someone might see it."

At that moment, a switch flicked. After watching and hearing John Edwards practise a thousand little deceptions and tell a thousand different lies, ostensibly in the service of some greater good, I finally recognised that he didn't care about anyone other than himself. A precious living, breathing human being – his daughter – had come into the world, and he wasn't inclined even to call the woman who had given birth to her. Shortly after, he asked if I could get a nappy to send for a DNA test. At one point, he even asked if we could get a fabricated DNA test, showing he was not the father.

Cheri and I finally returned, and Rielle's escape was arranged, but still Edwards would not own up. In 2009, Elizabeth published another book. In it I was no longer the young man she called "family". I was some sort of deranged groupie or "obsessed fan". I was hurt, but I didn't have time to dwell on it. The FBI and federal prosecutors had been to my home to talk about an investigation into allegations of corruption in the Edwards campaign. I began putting together the records they requested for a grand jury that had been convened.

Two weeks before my book was to be published in the US, John Edwards finally acknowledged his fifth child. This announcement fulfilled the promises he made to me, but came much too late to undo the damage done to me and my family.

My last encounter with Edwards had been in August 2008. I had no job prospects and our unfinished house was threatening to bankrupt me, but he was more concerned with his own problems. I asked him what he was going to do about me now that my reputation was trashed and I had no chance of finding a job.

"What about your promise to take care of things?" I asked. "What about you coming clean?"

"If you apply for a job, I'll give you a good reference. Just let me know who to call."

After a decade of devoted service, untold sacrifice on the part of my wife and children, and an act of extreme loyalty that left my reputation ruined, John Edwards proposed to compensate me with a good reference. As he drove away, I stole a last glance through the window of the driver's door. I saw a man I couldn't recognise at all.

This is an edited extract from The Politician, by Andrew Young, published by Thomas Dunne Books, at £16.59. © Andrew Young 2010