She even posed with Dos Santos and said she was "honoured to share this show with the President of Angola", reported Daily Star.
The Human Rights Foundation claims that by performing at a recent benefit for the Angolan Red Cross, the singer was accepting "dictator cash".
It is not the first time Carey, 43, has performed for a hated dictator. In 2008 she was paid USD 1 million to sing for then Libyan leader Muamar Gaddafi.