"As a singer, songwriter and producer, it's going to be fun and rewarding to help find new talent and give back with American Idol," Carey said in a statement released by Fox.
"I'm currently in the studio working on my new album and its first single, 'Triumphant,' which will be out early next month. I can't wait to channel my creative energy as a part of this show which is a massive global phenomenon."
"We are proud to have one of the world's greatest-ever female singers join our show," said Idol creator and executive producer Simon Fuller. "Mariah defines the word 'Idol' and will inspire every singer that has the honor of performing in front of her."
Recent reports claimed Carey was in the process of finalizing a deal to join Idol and asking for a salary of at least $17 million. Rumors she had been in talks with the Fox reality singing competition have circulated for weeks.
"Mariah Carey is a global icon whose list of musical accomplishments is unprecedented," added Idol executive producer Cecile Frot-Coutaz. "Having sold more than 200 million records worldwide, she's a true artist, ideally suited to identify and nurture American Idol's next great music superstars. We warmly welcome her to the Idol family."
"Mariah Carey is the real deal. Not only is she one of the best-selling artists and performers on the planet and one of the world's most gifted vocalists, she also is a successful songwriter, record producer and actress," said Fox reality chief Mike Darnell.
"She has a deep knowledge of music and a career insight that will make her a really amazing asset to our judges' table and to our contestants. It's the dream of almost everyone who auditions for Idol to sing like Mariah and we couldn't be happier to have her be part of the show."
Earlier this year, reports suggested Carey was also under consideration to become a judge on this fall's second season of The X Factor, which will feature Britney Spears and Demi Lovato as its new judges alongside returning panel members Simon Cowell and L.A. Reid. However, Carey's husband -- America's Got Talent host Nick Cannon -- quickly came out and bashed the idea.