The Fireflies star reveals the bands recorded their parts on the track separately, and although they had communicated over the phone and via email, they had never actually met until this week, on the eve of their first TV performance together.
Appearing on U.S. breakfast show Today on Tuesday (14Jul15), Young confesses, "It's a long time coming, but it felt like it was meant to be, at long last."
MMMBop singer Taylor Hanson adds, "We were working remotely. It's a modern romance, a musical romance."
And Young admits the idea to recruit pop trio Hanson for the tune was somewhat of a last-minute decision as he recorded his latest album, Mobile Orchestra.
The star says, "It was sort of the 11th hour of the demo and I sent it to the guys and they just made it far better than it would have been otherwise."
The collaborators didn't even need to meet up for the music video for Unbelievable - the colourful promo for the 1990s flashback tune was all animated.