A spokesperson for the 45-year-old star confirmed the engagement had taken place on Thursday evening in New York.
Casino owner James, 47, who has been dating the mother of two since last summer, arranged for a private dinner at the Eleven Madison restaurant in the city surrounded by her closest family and friends, where he popped the question.
He presented her with a ring that's reportedly made of a 35 carat diamond.
This will be the third marriage for both James and Mariah - the singer split from her ex-husband record producer Nick Cannon in 2014.
The five-time Grammy award winner has four-year-old non-identical twins from her marriage to Nick, Moroccan and Monroe.
Moroccan and Monroe
Business mogul James has three children of his own – Indigo, seven, Jackson, five and Emmanuelle, three, from his second marriage which ended in 2007.
The celebrity pair first met at an exclusive ski resort in Aspen, Colorado at a film premiere for Hercules.
Mariah, whose net worth is estimated at $500 million, said; 'We were talking and laughing and people were getting mad at us and stuff like that. So we hit it off.'
Several months later they officially confirmed their couple status while on a sailing holiday on the Italian island of Capri on James' private yacht.
Mariah was clearly smitten when she told Australian fans at a concert in Melbourne; 'Spectacular, handsome... I don't even have the words for the man who introduced me tonight. We'll just say the amazing Mr James Packer.'
Mariah and ex-husband Nick Cannon
Between 1993 and 1998 Mariah was married to music mogul Tommy Mottola. No further details about a wedding date have been released yet.