"R.I.P my Angel," Johnson's fiancée, the reality star Tila Tequila, wrote Monday. "u will forever be in my heart! I love u so so much and we will Marry when I see U in Heaven my Wifey."
She later added: "I can't stop these haunting visions of her and I. We made such a lovely couple, only beginning to spend the rest of our lives together…"
The Hilton sisters, who were close friends of Johnson's, also mourned publicly.
"In bed crying, looking at baby pictures of Casey, Nicky and I," Paris Hilton wrote on her Twitter page. "I feel so upset. I feel like I've lost a sister. My heart is broken."
Nicky Hilton, who was the godmother to Johnson's adopted daughter Ava, wrote: "Rest in peace Casey Johnson. I love and miss you so much. You are my best friend, my sister...I am broken without you."
Lindsay Lohan and Samantha Ronson also Twittered their feelings of loss. "R.I.P. Casey Johnson," Ronson wrote. "Am so sick of those 3 letters."
Lohan added: "r.I.p. To a friend to and of many- Casey Johnson- my regards to her family, friends and her beautiful daughter Ava* you are in our hearts CJ."
Police were called to a house in Los Angeles on Monday morning, where they found Johnson dead at the scene. There was reportedly no evidence of foul play. A toxicology report is forthcoming.