The quest to crack America has driven many a British band to breaking point, but Mumford & Sons are one of a kind. The multi-platinum quartet’s lofty worldwide standing has been underpinned by astonishing success across the pond. So why have they flourished where others floundered?

“We were willing to play a lot of shows, visit radio stations, meet people and spend a lot of time there. Lots of people aren’t,” muses frontman Marcus Mumford, matter of factly. “Our lawyers ...