While Prince Harry earnestly pledges to combat real suffering with a song to raise awareness, Sheeran agreed: "People just don't understand what it's like for people like us."
As the musician referred to "snide comments" and "jokes" that had to be stopped, it dawned on the Duke that they were talking about very different causes.
"We are ginger - and we're gonna fight," said Sheeran, before the Duke revealed the "slightly awkward" misunderstanding that he was actually hoping to challenge the stigma of mental health.
The skit saw the pair, two of the most famous redheads in the world, take the spotlight as amateur actors.
At the end of the video, posted for their fans on Instagram, the Duke reverts to his real self to urge viewers to seek help for mental health issues and not "suffer in silence".
The video had been trailed by the Duke and Duchess of Sussex's official social media feed a day early, in the hopes of leaving followers eagerly awaiting the big reveal.
The final video, which was one minute and 40 seconds long, opened with Sheeran having his beard trimmed in preparation for meeting the Duke.
While it purported to see the Duke welcome a camera crew into his own home, it is understood that the film was not made in Frogmore Cottage.
Avid royal-watchers identified the house instead as Ivy Cottage, where Princess Eugenie and Jack Brooksbank live at Kensington Palace, comparing the front door with an old image of the building and noticing a large wedding photograph of the pair mounted in the hallway.
"I'm really excited today," Sheeran said as he introduced the video. "Going to film a thing with Prince Harry. He contacted me about doing a charity video with him which you know is going to be good. I've long admired him from afar."
As he greets Sheeran at the door, the Duke notes of his appearance: "It's like looking in the mirror."
Talking about their shared "passion", the Duke said: "This for me is just something that's just not talked about enough, and people all over the world are really suffering."
Told that Sheeran had "actually been trying to write a song about this to get it out to more people", he added: "I was about to say, someone with your skillset to be able to write lyrics on a song to raise awareness, that would be amazing.
"That's exactly what I'm trying to do," said Sheeran. "People just don't understand what it's like for people like us. With the jokes and the snide comments.
"I just feel like it's time we stood up and said 'We're not going to take this any more. We are ginger - and we're gonna fight'."
Prince Harry tells Sheeran: "Slightly awkward. This may be a miscommunication but this is about World Mental Health Day?" Photo / Instagram
Pretending to realise they were speaking at cross purposes, the Duke interrupted to say: "Slightly awkward. This may be a miscommunication but this is about world mental health day?"
Sheeran, insisting he knew all along, was then filmed deleting the words "Gingers Unite" from a presentation on his laptop, along with the subheading: "HRH Prince Harry and the king of ging Ed Sheeran get together to change the perception of people with Moroccan sunset hair."
Reverting to the real selves, the footage ends with the pair sitting side by side on a sofa as the Duke delivers the message: "Guys, this World Mental Health Day, reach out, make sure that your friends, strangers - look out for anybody that might be suffering in silence. "We're all in this together."
The clip was captioned: "It's World Mental Health Day!
"Both Prince Harry and Ed Sheeran want to ensure that not just today but every day, you look after yourself, your friends and those around you.
"There's no need to suffer in silence - share how you're feeling, ask how someone is doing and listen for the answer. Be willing to ask for help when you need it and know that we are all in this together."
