The 'Obsessed' hitmaker has signed up to join a selection of the world's biggest stars and add her vocals to the music mogul's star-studded cover version of the REM ballad 'Everybody Hurts'.
Simon has revealed the 40-year-old singer-and-actress was so keen to help raise relief funds for the Caribbean island following the two devastating earthquakes, she altered her schedule to fit in the recording.
He said: "We're getting there. We have had an incredible amount of support. It's amazing these great singers are clearing their schedules to make this happen. I just found out Mariah has changed her schedule to be able to do this too.
"It says such a lot that everyone is working so hard to help Haiti. This is something that really matters to us all and I'm so grateful for the support we are getting."
Simon - who hopes the song will be released next Sunday (31.01.10) - is thrilled about the extra publicity which will be generated now Mariah has joined the bill.
A source added to The Sun newspaper: "Mariah was brilliant, she wanted to help in any way she could. Her name gives the single another great boost."
'Hannah Montana' star Miley Cyrus has also joined the track's all-star cast.
The song is particularly poignant to JLS member Oritse Williams, who has lost family members in Haiti.
He said: "This tragedy is very close to my heart. We hope to raise as much money as possible."
The individual vocals are being compiled by producer Steve Robson, using advanced technology which means artists do not need to record together.