Mumford is featured on the 2014 collaborative album Lost on the River: The New Basement Tapes, with other artists including Elvis Costello, Rhiannon Giddens, Taylor Goldsmith, Jim James, Jay Bellerose, and T Bone Burnett.
Mumford takes co-writing credits on tracks, "Kansas City," "When I Get My Hands on You," "The Whistle Is Blowing," "Stranger," and "Lost on the River #20." In 2014 Mumford produced Hold Fast by Christian Letts (Edward Sharpe and the Magnetic Zeros), released February 2015.
He began his musical career playing drums for Laura Marling on tour, along with the other current members of Mumford & Sons.
It was through touring with Marling and gaining experience playing gigs, as well as experimenting with his early writing that they decided to form the band in 2007.
The source said: "Carey's hoping they can spend more time together when Marcus flies back to America next month for a major tour with his band.