The Fox network officially welcomed the pair with an announcement on Sunday, just hours before the first round of judging for next season was due to begin in New York.
The announcement also settles the status of Randy Jackson. He will stay put as the sole remaining original Idol judge, scotching rumours of a different role for him on the popular talent competition.
This trio joins newcomer Mariah Carey on a judges' panel now expanded to four members from its previous three.
American Idol returns for its 12th season in January.