Fox Entertainment President Kevin Reilly revealed the appointment at tthe Television Critics Association summer tour in Beverly Hills as he was speaking about the duo's exits.
He said: ''We do, in fact, have one judge whose deal closed very, very recently. It's 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.''
Mr. Reilly then put Mariah on speakerphone to the room.
She said: ''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.''
The future of the only original member of the panel, Randy Jackson, is as yet unclear.