Talking to The Huffington Post, the Deer in the Headlights author confessed that Palin is not the person he once knew and has become considerably more vain since earning money.
"It's not the Bristol I once knew. After she got the fame and the money it all changed after that. I think she definitely changed into the worst," Johnston said.
He also commented on recent rumours that Palin had undergone plastic surgery after a "necessary" jaw operation changed her face dramatically.
"She looks like she got some jaw work going on there. She was uncomfortable with [her body] growing up but was never one to go out and get plastic surgery. I mean, she never wanted to do that. But now she's got the money," Johnston explained.
Palin recently accused Johnston of being "desperate for attention" for saying negative things about her to the press.