"I have my own reasons for being involved in mental health: What happened to me and my mother when I was younger", he said. It's the same as physical health.
He revealed that losing his mother on such a "public platform" affected not only his personal and public lives, but also his mental health.
"It's interesting to see and hear from you how much having that conversation. has really made a difference to you", William said in the video.
A source tells Us Weekly that the Duke of Cambridge, 34, "chose to first contact Lady Gaga after she bravely opened up about PTSD". "It was only three years ago that, from the support around and my brother and other people around who started to say, 'You need to deal with this".
It was the latest stage in a campaign by the royals to raise the profile of mental health.More news: French police thwart attack days before election
William praised grime musician Stormzy's revelation about his depression as "incredibly powerful", saying: "There may be a time and a place for the "stiff upper lip", but not at the expense of your health". The 32-year-old prince acknowledged that he spent almost 20 years "not thinking" about the death of his mother, Princess Diana, and that he only got help after two years of "total chaos".
Gaga - whose full name is Stefani Germanotta - spoke about her struggle with PTSD in an open letter for the first time in December past year, and she believes addressing her internal fight helped her to overcome her battles, and she has encouraged other people facing the same problem she did to speak about it.
The film was recorded on Facetime between their homes in Los Angeles and London as part of the Heads Together campaign.
William told Lady Gaga in the clip, "Harry, Catherine and I really felt this was such an important area that throughout all our charitable work, whether it was the veterans, homelessness, addiction, most of it seemed to stem back to mental health issues". "There's a lot of shame attached to mental illness - you feel like something's wrong with you". You won't be judged.
"It's so important to break open that fear and that taboo", Prince William says. "I think it's just a tragedy that we're not talking about this at all. That is what we need to do". Prince William did invite Lady Gaga to London to participate in a Heads Together project with him.