"She was absolutely head-over-heels in love with him."
In his book TMI: My Life in Scandal he revealed he met Mayer and Simpson at nightclub in 2007.
"I'd never met John before, but the minute Jessica introduced us, he began chatting to me. It was almost like he had been waiting to meet me," Hilton wrote.
"Suddenly, he leaned in to me and said, 'I like to watch gay porn, you know.' "
Hilton was stunned into silence as Mayer talked about his favourite gay porn star and how the man "really turns me on."
"Everything happened very quickly after that," he wrote.
"John leaned in close and pushed his tongue into my mouth, and before I knew what was happening, he was full-on making out with me."
As this happened, Hilton wrote, Simpson stared at them, "frozen," before deciding to join in.
"She blushed when our eyes met, and quickly covered her face with her brunette hair," Hilton writes. " … John groaned quietly as he kissed me, but I was just trying to work out what was going on."
In a later interview with Playboy, Mayer said it was an act, saying that he thought Hilton was "acting as though he had just invented homosexuality."
"All of a sudden I thought, I can outgay this guy right now," he said in the interview.