"Hey mate, nice to see you," says Harry.
"It's like looking in the mirror."
The video goes on to encourage viewers to visit the Every Mind Matters website for "tips on coping with life's ups and downs."
The Instagram clip is believed to be the first time senior royals have appeared in an advertisement.
Earlier this week, The Duke of Duchess of Cambridge and the Duke and Duchess of Sussex reunited to help voice a "special film" about mental health in the United Kingdom, just months after famously parting ways from their joint charity.
William, Kate, Harry and Meghan rejoined forces to narrate a three-minute film for the National Health Service (NHS), encouraging Brits to look after their mental health.
"(Every Mind Matters) will offer vital support to those living with a mental health condition and give each of us valuable and personalised tips on how to better cope with life's daily struggles, while contributing to tackling the stigma that can still surround mental health," UK health secretary Matt Hancock said.
A teaser posted online by Kensington Palace, featured Prince William explaining how life could get people down.
"Everyone knows that feeling, when life gets on top of us."
Prince Harry continues: "We feel stressed, low, anxious or have trouble sleeping. We think there's nothing to be done, nothing we can do about it."
Then Meghan chimes in: "But now, there's a new way to help turn things around. Every Mind Matters will show you simple ways to look after your mental health".
Next Kate explains how the campaign can get people started "with a free online plan designed to help you deal with stress, boost your mood, improve your sleep, and feel more in control".
Then Prince William concludes the ad by saying: "We can all benefit from taking simple steps to look after our mental health".
