Black Film reports that the new American Idol judge will reunite with her Precious director Lee Daniels on the film, which is a biopic of White House butler Cecil Gaines (Forest Whitaker).
Carey made a famously de-glamourised appearance in Daniels's acclaimed 2009 movie, playing a social worker who supports the film's troubled protagonist Precious (Gabourey Sidibe).
In addition to Whitaker, The Butler will star John Cusack as Richard Nixon, Alan Rickman as Ronald Reagan, Jane Fonda as Nancy Reagan, Robin Williams as Dwight D Eisenhower, James Marsden as John F Kennedy, Minka Kelly as Jackie Kennedy and Liev Schreiber as Lyndon B Johnson.
Matthew McConaughey, who stars in Daniels's controversial Cannes contender The Paperboy, was originally attached to play JFK but was replaced by Marsden after dropping out last month.
Production on The Butler, which is written by Daniels and Danny Strong (Recount, Game Change), began this week in New Orleans.
The Butler will be released in 2013. Watch a clip from The Paperboy below: