Marcus Mumford

In this video source, you can see Marcus Mumford with a Martin D-28. He was also mentioned on the Martin Guitar Ambassador page. The July edition of Guitar Buyer has an article on Marcus' guitars and setup and includes two Martin D-28s are called "Mary-Kate" and "Ashley".

Marcus Mumford frequently uses the Martin OM-21 acoustic guitar during live performances and in various videos, including "Little Lion Man" and "White Blank Page (Bookshop Session)." In a live performance of "Reminder" on 89.3 The Current, he prominently features this guitar, highlighting its significance in his musical repertoire.

Marcus was on the hunt for a new (to him) Fender Telecaster, something with some Springsteen vibe. After picking and playing several iconic teles, he stumbled across this really unique piece that just found its way to Chicago Music Exchange two days prior. It’s a 1967 Fender Telecaster in a blonde finish. What really sets this guitar apart from the others is the modified pickup in the neck. The original pickup was replaced with a vintage Gibson Patent Number pickup that really gives this guitar a screaming rock n’ roll sound. Source: https://www.chicagomusicexchange.com/blogs/soundboard/11878125-mumford-sons-talk-vintage-guitars-at-chicago-music-exchange

Marcus uses this guitar a lot on Wilder Mind and it joined him for all of the Wilder Mind tour.

In this video, Marcus can be seen playing a Collings D3 dreadnought at the One Love Manchester concert. The Collings D3 dreadnought has a similar design to the Martin D28, and Marcus began using it back in 2011 interchangeably with his Martin D28 (Likely multiple D28s). At the 2011 Grammy awards, Marcus played his Collings D3 during Mumford & Sons' live performance of The Cave.

At 0:09 in this video, Marcus Mumford holds a Fender Jazzmaster.

In this video, at 0:11, one can see Marcus Mumford playing a Fender Telecaster on the left side.

He uses this electric guitar for live performances of "Bellow my Feet". In the video of their Letterman performance he can be seen using one,

As far back as 2011 Marcus could occasionally be seen playing a Gibson ES-335 Semi-Acoustic guitar, as at the Bonnaroo music festival:

He plays often this guitar, even if it's not his main guitar, you can see he plays with this guitar in many live songs.

According to Collings artists page Mumford plays a C10 in Sunburst. In the picture he can be seen with one on stage.

In the video you can see Marcus playing a Gibson 1968 J-50 with The New Basement Tapes.

Seen using his Gibson LG-0 in this live video