The new track has fuelled speculation that the band will soon release their follow-up album to 2009's Sigh No More.
The folk group have played previously unreleased tracks during summer festival gigs, but the new track is the first to receive a high-quality recording.
The 'Timshel' band recorded a session at Colorado radio station KBCO. Although it has yet to be given an official title, the track has been tentatively named 'Home' by fans online.
Sigh No More passed the million sales mark in both the UK and the US in March.
Listen to Mumford & Son's new track below: