The couple, who have enjoyed a whirlwind romance since they started dating in June 2015, attended the G'day Gala in Los Angeles on Thursday.
It gave the pair a chance to show the world just how in love they are - not to mention flash that super expensive engagement ring.
Mariah had no problem flaunting the 35-carat sparkler - believed to be worth around £8m - as she smiled by the side of her man.
more photos after the cut
Meanwhile billionaire businessman James was hardly letting the side down.
The 48-year-old looked dapper in a suave black suit teamed with a matching tie and crisp white shirt.
The pair appeared absolutely absorbed in one another, sticking close and whispering between them as photographers snapped away.
A source told TMZ: "Mariah is just beyond excited right now.
"She has wanted to be his wife pretty much since they first got together, and has already started to plan what type of wedding she wants to have."
Located in an upmarket gated community, the 18,000-square foot house features a sauna, a gym and is set over one acre.