The star was asked to step in to replace the folk rockers after they pulled out of the show so bassist Ted Dwane can recover from surgery to remove a blood clot from his brain.
Johnson has now revealed he was already en-route to the event when he received a phone call offering him the coveted headline slot, and the Upside Down hitmaker couldn't believe the coincidence.
He says, "I was on my way to Bonnaroo to sit in on a set with my friends (band) ALO... when we got the call asking if I could possibly help out and cover for Mumford & Sons. I heard the news about Ted Dwane and was happy to know that he was on his way to recovery. I called my band and asked if they were up for it."
Johnson headlines the event on Saturday night (15Jun13).