America was stunned when “poor little rich girl” Casey Johnson, the troubled first-born daughter of billionaire New York Jets owner Woody Johnson, was found dead at age 30 in January 2010.

The Band-Aid heiress had been cut off from her fortune and family, whose help she refused. Her death from complications of diabetes sadly ended a short but scandalous life.

Now, Casey’s turbulent world is ex-amined in JERRY OPPENHEIMER’S new bombshell unauthorized biography, “Crazy Rich: Power, Scandal, and Tragedy Inside the Johnson & Johnson Dynasty.” Casey’s socialite mother, Sale Johnson, along with relatives and friends, spoke candidly for the first time since her death. Now, in an exclusive excerpt, Oppenheimer paints an astonishing picture of the doomed heiress and the role her mother and father played in their daughter’s life and tragic end . . .

Ten-year-old girls fantasized about fashion with their Ken and Barbie dolls, but when Casey Johnson was that age she dressed up for real with her first, but not her last, Chanel bag. At 11, she donned a pair of snakeskin pumps. Even though she didn’t have a driver’s license, Casey was given her own car at 16. At 18, she got breast implants. “I got whatever I wanted,” she once boasted.

“Woody over-indulged Casey,” a family member asserts. “That was Woody’s way. He was raised with the idea that money can do everything, and that’s what worked for him.”

When Casey was 9 years old, she became increasingly volatile and disruptive. One of several psychiatrists who Casey saw throughout her life became a father figure because, according to her mother, Woody could not relate to his wayward child.

Her mother asserts, “Woody was not a warm, cuddly kind of person. With Casey, Woody was so uncomfortable because he didn’t know what to do with her, or how to react to her situation because she was not easy to deal with. She was very complicated, and it was overwhelming in a large part for Woody despite his best efforts.” Adds Johnson: “All Casey wanted was her father’s approval. She lived for that, and she was broken down because she didn’t get it.”

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Casey was diagnosed with diabetes in 1988 at the age of 8, but her growing disruptiveness at home and school was not related, as first assumed, to her physical condition.

In adolescence, it was determined that she had borderline personality disorder — a serious mental illness marked by unstable moods, behavior, and relationships — symptoms that grew increasingly severe in the last years of her life. “Borderline personality disorder ruled Casey’s life,” declares Sale Johnson, revealing her daughter’s mental condition for the first time. “It stole her teenage years and her young adulthood life away from her. It’s a mental health disease that confounds, scares, hurts the victim, her family, her friends, and her doctors. They don’t want to treat it because it has the highest suicide rate, and no cure, and [someone like Casey] is a 24/7 patient.”

As Casey got older, gossip about the pretty heiress’s aimless lifestyle and extravagant ways spread across New York. As usual, Daddy was there to clean up the mess with his wallet.

Casey loved dogs since childhood, and considered them her “babies,” but they would make incredible and costly messes, including even in her $12,000 Hermès Birkin bag, where she carried a teacup pooch everywhere. In the fall of 2005, she was staying in a luxurious suite at the Plaza Athénée in Manhattan, when her Chihuahua, Tukus, had the runs, and defecated everywhere. Woody was forced to foot the clean-up bill, said to be as much as $20,000.

Throughout her teens and 20s, hard-partying Casey drank and, according to her mother, did recreational drugs to ease her emotional pain. She lasted just one semester at Brown University, and constantly neglected her health. In 2001, the year her parents divorced, she moved to Hollywood.

Initially, she had a fantasy about a show business career (she’d taken singing lessons since she was 12), but mainly she just wanted to get away from her family. That was underscored by a story she once told about attending a Hollywood party where she overheard one girl telling another, “ ‘Oh, that’s the Johnson and Johnson girl,’ and my heart just sank because I don’t want to be identified like that. I’m Casey Johnson. I’m not the Johnson and Johnson girl. It really hurt.” Like so many members of the Johnson dynasty before her, she was wary of people, and felt some took advantage of her because of her name and wealth. “I’ve learned that the hard way. I’ve found a lot of people use me . . . I just let things happen, and then I find out, ‘Oh, my God, they’re totally taking advantage of me.’ ”

In 2005, her parents traveled to LA to stage the first of a series of interventions to persuade the increasingly emotionally disturbed Casey to enter rehab.

“We got there,” reveals Sale Johnson, “and Casey just blew us off. She said, ‘I don’t need any help. I’m sorry you wasted a trip.’ After that, Woody basically washed his hands of Casey.”

In March 2006, Casey, then 26, very publicly fell out with her five-times married aunt, Woody’s sister, Elizabeth Ross “Libet” Johnson, then 56, and excoriated her in the press, accusing her of stealing a boyfriend. The Johnson clan was mortified by the media coverage.

To the very private Woody Johnson and the Johnson dynasty as a whole, Casey was now considered a tabloid terrorist and her act of vengeance their own personal 9/11. Woody, who had mostly washed his hands of Casey because of how troublesome she was, cut off all ties with her, including her trust fund millions in a move of tough-love.

Meanwhile, Casey fell in love with the idea of adopting a baby.

“I told her I was totally against the adoption,” her mother emphatically maintains. “I said, ‘You don’t have your own life together, how are you going to keep track of somebody else’s life? This is not a puppy that if it doesn’t work out, you can give it to a friend.’ ” Casey had never planned to have a child of her own, Sale Johnson says, because she was aware, when lucid, of her mental instability from borderline personality disorder and poor health as a result of her diabetes. But in 2007, against her divorced parents’ wishes, Casey adopted a Kazakh baby girl and named her Ava-Monroe, in honor of her idol Marilyn Monroe.

The following year, a hysterical and hurtful family confrontation was ignited involving Casey, Woody and his much younger future second wife, Suzanne Ircha, at the Jets owner’s Hamptons estate.

During one of her up periods, Casey had come east with hopes of introducing her father to Ava-Monroe, and ending their long estrangement. By the time Casey showed up on her father’s doorstep with two-year-old Ava in tow, Woody had been incommunicado for several years. Woody wasn’t home, but Ircha came to the door. Casey let everyone know her father’s girlfriend was far from hospitable. “What are you doing here?” Ircha fumed, Casey later told her mother.

When Casey explained that she had come to see her father, Ircha was said to have replied, “This is my house, so leave.” But Casey stood her ground. “This is my father’s house and I’m staying here until he gets here because I want him to meet my daughter.”

Words flew, and Ircha dialed 911. About the same time that the police arrived, Woody pulled up, and demanded that his daughter get off of his property, stay off, and never come back. [Woody Johnson declined to be interviewed for Oppenheimer’s book.]

“Woody doesn’t like confrontation. He doesn’t like negative publicity. He doesn’t like anything like that,” maintains his ex-wife, Sale, of the incident. A close relative recalls a conversation with Casey not long after the contretemps. “I said, ‘Well, how are things,’ and she said, ‘My fondest wish, my dearest wish, is that I can someday be on good terms and talk with my father again.’ I was shocked. I said, ‘Casey, what are you telling me?’ And she says, ‘He won’t have anything to do with me. If I go to his house he tells me to get off his property.’ It was really heart-wrenching.”

In the last couple years of Casey’s life, her mother claims, “Casey sent love letters to her father. She called and left voice mails, and Woody chose not to respond.”

In June 2009, six months before her death, relations soured between Casey and her mother, who was desperately fighting to get her into treatment for her mental disorder.

While Casey was scheduled to be hospitalized that day at the luxurious Cliffside Malibu clinic, her mother planned to take three-year-old Ava back to New York, and care for her while Casey was in treatment, but she had promised to bring her back for visits. None of it happened the way Johnson envisioned. Before the day had ended, Casey had thrown her mother out into the street, luggage and all, and called the police claiming she was trespassing and attempting to take her baby. “Casey knew in her heart that she couldn’t take care of Ava, but she couldn’t ego-wise and illness-wise say: ‘I know I can’t take care of her like she needs to be cared for …’ ” says Johnson, recalling that awful day.

During that horrific time, Sale Johnson felt bitter about her ex-husband’s lack of involvement with his daughter.

“He didn’t want to have anything to do with Casey,” she says. “It was too much trouble. But fathers are supposed to take a bullet for their kids, and he went the other way. I can’t defend his behavior for that because I thought it was appalling. But that’s who he is. He doesn’t have the emotional makeup to deal with it. It’s like: ‘I’ll be an ostrich and put my head in the sand, and when I pull it out, everything will be good.’ ”

After Casey died, Woody told a reporter that his long-estranged daughter had been “trying to find her own identity. She was rebellious. She made some judgment errors. Been there, done that. She had to take responsibility. And it couldn’t be me pushing. Or her mother. Or her doctor. She would ultimately have to do it herself.”

Her mother eventually persuaded Casey to temporarily entrust her with Ava’s care while she was hospitalized for her diabetes in August 2009.

The visit was believed to be the last time that she saw her daughter alive. Meanwhile, released from the hospital, Casey moved back into her rented Beverly Hills home. But by the fall of 2009, her family, still practicing tough love, had stopped paying Casey’s rent. With her money cut off, Casey was getting deeper into debt. Her Porsche was repossessed, a former landlord sued her for back rent and property damage, and other bills for her extravagances piled up. Facing eviction, Casey found new lodging in the luxurious, gated, and private two-bedroom West Hollywood guest house owned by one of her mother’s friends. It was her final stop.

In the last weeks of December 2009, in one of her more outlandish, headline-making episodes, Casey made public a bizarre romance with the bisexual reality TV personality and exhibitionist Tila Tequila, the two kissing for the paparazzi.

Tequila boasted that they were going to get married, and that Casey had given her a rock of an engagement ring. She called Casey her “Wifey.” Back east her father and other members of the Johnson dynasty cringed.

That Christmas and New Year’s, after a bizarre incident in which she was arrested for the alleged burglary of a friend’s apartment and with Tequila out of town, emotionally fragile Casey stayed alone in the guesthouse.

She had all but stopped taking her insulin, she was eating junk food, and swigging NyQuil in order to sleep. She also had started communicating via Twitter and Facebook with her friends and those in the outside world who were following her increasingly sordid real-life soap opera that was being played out in the tabloids and on-line. Her final one said, “Sweet dreams everyone . . . I’m getting a new car . . . Any ideas? Cant be a two seater cause we have a daughter . . . sedan, sports car, suv??”

From what was known, she spent New Year’s Eve, when she usually was out partying, alone in bed. Around 11:30am on January 4, 2010, when Casey didn’t respond to knocks on her door, she was found unconscious. Shortly before noon, Pacific time, a 911 call was placed by an unidentified female from Casey’s residence. “She’s ice-cold and her hands are turning blue,” stated the caller. “I have two other people here with me and we all think she’s dead. I don’t know if it’s suicide. Very often her medication gets all screwed up, so it’s probably that.” Paramedics arrived shortly thereafter. The Johnson & Johnson heiress whose life had been both a Cinderella fantasy and a living hell was pronounced dead on the scene.

It was a needless tragedy brought on by Casey’s dual illnesses — and her reckless approach to both.

Casey had everything money could buy and subsequently ignored all the rules about her diabetes diagnosis. “She thought she was invincible,” observes a friend.“Casey had always done whatever she wanted to do. She wound up in the hospital a few times, but the diabetes never killed her. I guess she thought she could do whatever she wanted to do — until the diabetes and her life-style did kill her.”

Sale Johnson remains devastated over the loss of her first-born daughter, but draws comfort from her hands-on approach to raising Ava-Monroe. The 7-year-old adores her adoptive grandmother, who earlier this year split from her second husband, NFL player-turned-sportscaster Ahmad Rashad. Grandmother and granddaughter now live together on a gated estate in South Florida.

“Ava’s the most beguiling creature on this planet,” declares Johnson. “She’s just a freak of nature. She’s just happy and smart and so up for anything. Life is an adventure.”

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Adapted from “Crazy Rich” by Jerry Oppenheimer. Copyright 2013 by the author and reprinted by permission of St. Martin’s Press, LLC.