Nick Cannon is Mariah Carey’s true fan.

The Wild 'nOut host has known about Carey’s bipolar disorder diagnosis for a long time, a source close to Cannon tells ET, and he’s proud of her for speaking out about the matter.

“Nick has known about Mariah's diagnosis forever,” the source says. “This isn't news to him. This was a big part of their relationship and even today, it's something he's mindful of.”

The 48-year-old singer, who co-parents 6-year-old twins Monroe and Moroccan with Cannon, recently revealed in an interview with People that she was diagnosed with bipolar disorder in 2001.

"Until recently I lived in denial and isolation and in constant fear someone would expose me,” Carey told the magazine. “It was too heavy a burden to carry and I simply couldn’t do that anymore. I sought and received treatment, I put positive people around me and I got back to doing what I love -- writing songs and making music."

“He's proud of her for coming forward and using her platform for good,” the source adds. “He loves her as the mother of his children and always wants what is best for her.”

On Wednesday, Carey took to Instagram to share a heartfelt message with her fans, writing: "I'm grateful to be sharing this part of my journey with you."

For more on Carey, watch the interview below.

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