The singer's Australian beau popped the question despite the pair having dated for less than 12 months.
Cannon, who was married to Carey from 2008 to 2014, is said to be slightly taken aback by the news considering the speed of their whirlwind romance, reported Hollywood Life.
"While Nick certainly supports Mariah and James' decision to get married, he's a little shocked. Surely, it was in the back of his mind that the mother of his children could tie the knot again," a source said.
Despite Cannon's feelings, he is said to be adjusting quickly to the idea, mostly because he knows it would also be beneficial for his and Carey's twins Moroccan and Monroe.
James is said to have proposed in Carey's hometown, with a stunning 35 carat diamond ring.