Johnson was diagnosed with diabetes as a child, but it was unclear if that had a role in her death.
Johnson's body was found Monday in the home where she lived behind big wooden gates bearing the name "Grumblenot" in a quiet neighborhood. There were no signs of foul play, police Officer Gregory Baek said.
But there were signs of trouble in tabloid tales of a life of privilege seemingly spinning out of control.
Johnson was facing criminal charges alleging she burglarized a friend's home, reportedly had her adopted daughter taken away by her mother, and once publicly accused her father's sister of stealing her boyfriend.
Johnson was the daughter of New York Jets owner Robert Wood "Woody" Johnson IV. He is also chairman of the Juvenile Diabetes Research Foundation and co-authored the book, "Managing Your Child's Diabetes."
Woody Johnson learned of his daughter's death a day after his team clinched an NFL playoff spot for the first time in three seasons.
Jets head coach Rex Ryan opened his press conference Tuesday by offering condolences to the owner.
"It's obviously a tragedy for Mr. Johnson and his family," Ryan said.
Casey Johnson had no role in the health care products company that her great-great-grandfather founded in 1886, even though she and her two younger sisters were heirs to the fortune.
The family's involvement with the corporation ended decades ago.
Like her friend and former high school classmate Hilton, Johnson's exploits were detailed in tabloid reports of partying and sexual escapades. Johnson became a staple of the New York Post's Page Six, the gossip column where she was mockingly dubbed the "baby-oil heiress."
She was one of the subjects of the 2002 Manhattan documentary "It Girls," but screen fame eluded her.
Johnson told Vanity Fair in a 2006 interview that her decision to turn down Hilton's invitation to be her co-star on "The Simple Life" reality show was "the stupidest mistake of my life."
In a Twitter posting Tuesday, Hilton said she was devastated by the death of her friend.
"In bed crying, looking at baby pictures of Casey, Nicky and I. I feel so upset. I feel like I've lost a sister. My heart is broken. Miss her," she wrote.
Johnson and Tila Tequila canoodled and announced they were engaged in a Web video last month. Tequila referred to Johnson as "my Wifey" in a Twitter posting after Johnson's death.
Johnson was recently arrested for investigation of stealing $22,000 in lingerie, mail, jewelry and other items from the Hollywood home of model friend Jasmine Lennard.
She pleaded not guilty last month to burglary and receiving stolen property, and had a preliminary hearing scheduled on Feb. 2, district attorney's spokeswoman Jane Robison said.
Johnson also was reportedly in a custody fight over her adopted daughter with her mother, Sale Johnson.
Casey Johnson adopted the child as a baby from Kazakhstan in 2007 and named her Ava-Monroe after her idol, Marilyn Monroe.
"I see a lot of similarities between us," Casey said about Monroe in the Vanity Fair interview. "Her life makes me sad. I don't think she was very happy.
"They thought she was some dumb blonde, and she wasn't. She was a smart, smart broad. And I think that sometimes people look at me and think, 'Oh, Casey Johnson, she's stupid, she's blonde, she's an heiress, blah, blah, blah."'