William said he found Lady Gaga's open letter about living with PTSD after she was sexually assaulted at 19 years old "incredibly moving and very fearless".
"I thought it was incredibly moving and very fearless of you to write down such personal feelings", the royal told Gaga during the call released on the Royal Family's Facebook page - "I wanted to ask you how you found speaking out and how it made you feel?"
"We will all go through tough times in our lives, but men especially feel the need to pretend that everything is OK, and that admitting this to their friends will make them appear weak", Harry told CALM in a joint interview with William.
She said once you started sharing, you found out that lots of people suffer from some kind of mental challenge."I should be so happy, but you can't help it when in the morning you wake up, you are so exhausted, you are so sad, you are so full of anxiety and the shakes that you can't barely think", she told 34-year-old Prince William.
Lady Gaga continues "there is a lot of shame attached to mental illness - you feel like there is something wrong with you".
He said: "As you can see, you know, I have my own reasons for being involved in mental health, what happened to me with my mother when I was younger, but equally the charitable work I do at the moment and the areas that I'm involved in, it all comes back to mental health".
During a live Facebook broadcast on Tuesday, Prince William and Lady Gaga joined forces for the Heads Together campaign. Gaga tells the future King of England.
Hot on the heels of his brother talking about his own mental illness, Prince William has FaceTimed with Lady Gaga to talk about hers.
"If you don't acknowledge how you feel it will only bottle up, and could reassert itself later as illness", Prince William told the magazine.
Despite the difficulty, speaking publicly made all the difference. "We are starting to talk", she said.
And the singer believes that sharing your mental health struggles is key to managing them. William has since pledged to ensure his children "grow up feeling able to talk about their emotions", saying that it has been taboo or weak to talk about personal issues for too long.
Speaking after the preview screening of a BBC documentary about a group of London Marathon runners with psychological problems, he also said the more "influential and very important" people open up about their "issues and their battles" the better.