In the memoir titled, "Me", Elton John also described MJ's company disturbing, reports "dailymail.co.uk".
Elton John developed friendship MJ early on in his career. He has now admitted that over the years, he became concerned about the way in which he started to "sequester himself away from the world".
"I'd known Michael since he was 13 or 14... He was just the most adorable kid you could imagine. But at some point in the intervening years, he started sequestering himself away from the world, and away from reality the way Elvis Presley did," he writes in the book.
The singer continued: "God knows what was going on in his head, and God knows what prescription drugs he was being pumped full of, but every time I saw him in his later years I came away thinking the poor guy had totally lost his marbles. I don't mean that in the light-hearted way. He was genuinely mentally ill, a disturbing person to be around."
Recalling a time he invited MJ to one of his parties, Elton John claims that the star wandered off and was found playing with his housekeeper's son, prompting him to deduce: "For whatever reason, he couldn't seem to cope with adult company at all."
Michael Jackson died in 2009 due to overdose of sedatives administered by his personal physician, Conard Murray. Earlier this year, the star was posthumously accused of child sexual abuse by two men who were friends with him as youngsters in the documentary "Finding Neverland".