The singer said she is "so excited" to be signing up for the TV talent show, after the departure of judges Steven Tyler and Jennifer Lopez at the end of last season.
"This kind of all just happened really quickly," mum-of-two Mariah revealed. "I can't wait to get started in the fall (autumn) ... and I will see you in January."
CLICK ON PHOTOS TO ENLARGE
The show will be back on the air for season 12 in January.
Fox entertainment chief Kevin Reilly called Mariah "the biggest recording artist that any of these (talent) shows has ever had", adding that she was the "artist that many of these contestants have tried to emulate".
The Grammy-winning pop star is receiving just shy of $18 million (£11.6 million) a year for a one-year contract with a renewal option, according to a person familiar with the negotiations.
The exit of Steven and J-Lo left original judge Randy Jackson and started a guessing game about potential replacements, with Mariah at the top of the list.
Simon Cowell, the former American Idol judge who adapted his UK hit The X Factor for Fox, was asked about Mariah's new gig at a critics' session.
"I'm happy for her, actually. I like Mariah. I think she's going to find it difficult to say no. You've got to say no to people and she's sweet," he said, then quickly backtracked, adding, "No, I think she'll be great."
Simon had reason to be diplomatic: Mariah's husband, Nick Cannon, is host of his America's Got Talent on NBC.