hnson became a staple of the New York Post's Page Six, the gossip column where she was mockingly dubbed the “baby-oil heiress.”
Johnson, Hilton and Hilton's sister, Nicky, were among the socialites featured in the 2002 documentary, “The It Girls.”
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.“’
Associated Press Writers John Antczak in Los Angeles and Dennis Waszak in Florham Park, N.J., contributed to this story.