The 42-year-old singer has joined the panel of the reality show following the departures of Steven Tyler and Lopez, reported New York Daily News.
Fox Entertainment President Kevin Reilly confirmed the news on the phone with Carey in front of television critics on July 22.
"We do, in fact, have one judge whose deal closed very, very recently. It's (Mariah) an artist that many contestants have tried to emulate, have tried to have sung the songs, have tried to hit the notes and because it really was concluded only hours ago, our judge couldn't be with us," said Kevin Reilly, President, Fox Entertainment.
Carey is reportedly the highest paid judge on any music competition show after signing an USD 18 million deal for it.
"I am so excited to be joining 'Idol'. First of all, I wanted to be there today and I wish I could've been there to tell you, but I think you know this all kind of just happened real quickly, so I can't wait to get started in a couple months," she said.
The future of the only original member of the panel, Randy Jackson, is yet unclear.
R&B star Rihanna and rapper Drake are leading the MTV Video Music Awards (VMA) nominations with five nods each.