The 'Hero' singer will replace Sinitta – the music mogul's ex-girlfriend and long-running helper on the UK version of the show – and advise the 51-year-old show chief when he chooses his finalists from the category he is given to mentor in the final stages of the competition.
However, 42-year-old Sinitta – who previously dated Brad Pitt – is not happy with the decision.
She told the Daily Star newspaper: "Well, let's just say feathers will fly if Mariah takes my spot. Although she is a mega diva vocally, you can expect another 'X Factor' USA drama. I won't just roll over and take it. I hope she can run in high heels."
In April, Simon spoke of his desire to have Mariah involved with the programme, but admitted he was unsure in what capacity.
He said: "I think Mariah will have a role on the TV live shows. I met her recently and she was on great form. And her idea was to be the judge of the judges, which only Mariah could come up with.
"She'll be involved in some form. I literally adore her. I love her to bits."
Traditionally judges in the UK show take potential finalists to a special overseas base, and while Simon has previously taken his to Los Angeles, it is thought he will head to Britain when the US version is shown.
The audition stages of the 'The X Factor' are currently taking place, with British singer Cheryl Cole sensationally sacked from the programme after taking part in just four episodes, to be replaced by Nicole Scherzinger.
LA Reid and Paula Abdul are also judges on the US show.