"Catherine, Harry and I are incredibly proud to be working with you all to tackle one of the greatest challenges facing our society," William, 34, told those gathered at London's County Hall near the Eye of London. "Mental health is not a dirty word. We all have mental health like we do physical health, good or ill. But not seeking help at those times when it all seems too much, or we are depressed or anxious, can impact the rest of our lives."
William added: "Put simply, the three of us want to make asking for help no longer a big deal."
The young royals are on a mission to make talking about mental health more acceptable in society.
"It's time we ended the shame around mental health, the fear of judgment that stops people talking or getting help," said Harry, 32.
Kate, who wore a pale pink floral dress by U.S. designer Kate Spade to the event, also took a turn speaking to the gathered crowd, encouraging them to seek help if they feel overwhelmed.
"All of us know someone who has been through difficult emotional times, and we know how hard it can be to see a way forward," Kate, 34, said. "William, Harry and I feel it is our duty to do what we can, with your help, to shine a spotlight on emotional well-being and highlight the support that is out there to prevent or manage the pain at difficult times."
Kate continued: "Over the coming months, we hope to explore what else we can do to increase the level of service and support that people can receive. But first, as William said, we must tackle the stigma that stops people asking for help in the first place. We want to encourage people to talk to one another."