Jack Johnson Is Bonnaroo’s Last-Minute Headliner…and Writes a Last-Minute Song About It!

Jack Johnson was the hero of Tennessee's Bonnaroo festival this weekend, when he swooped in and replaced canceled Saturday headliners Mumford & Sons with just two days' notice, and even performed an original song he wrote about the whole surreal experience just hours before he took the stage.

Mumford & Sons were sadly forced to bow out of their Bonnaroo appearance (the first headliner cancellation in the festival's 12-year-history) after M&S bassist Ted Dwane underwent brain-clot surgery only a few days before the fest. Jack, who just happened to be at Bonnaroo to hang out and jam with his friends in the band ALO, was asked by festival organizers to be Mumford's emergency fill-in, and — despite not playing 'Roo since 2008, and not even playing a full set with his band in two years — agreed to save the day.

"We practiced for a couple of hours," Jack joked with the crowd when he and his band members, whom he'd only flown in the day before, hit the main stage at 9:30pm on Saturday night.

Jack's cover of Mumford's own "The Cave," in honor of his absent peers, was well-received by the appreciative crowd, but fans gave him mad respect when he performed a funny little ditty called "It's Bonnaroo," which he'd penned Saturday morning. Sample lyrics: "I had a late-night gig with ALO/It was very low-stress, it was very low-pro/But then the phone rang and things got strange/And my low-pro was about the change/'Can you get the band together in two days to play a show up on the big stage?'/But I don't know, it's been a year or two since we played these tunes/What the hell, it's Bonnaroo!"

Said Jack, "We want to dedicate this whole set to Ted Dwane and all of Mumford & Sons, and we wish we could all be here playing music together. Maybe that will be next year." We'll see if that happens, but for now, it's safe to say that Jack was definitely the star of this year's Bonnaroo.

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For more Bonnaroo reportage, check out Yahoo! Music's 2013 Summer Music Guide.