Around 100 people, including some from the United States, were turned away from the band's Portsmouth Guildhall gig in the UK on Thursday night (November 22).
A fan named Paula Wynne told BBC News that she and her partner had purchased their invalid tickets for £200 from resale website Viagogo.
Viagogo has announced that victims will be offered real tickets to another Mumford & Sons concert, or a voucher to the value of the amount the purchasers paid.
Following reports that some of the fraudulent tickets had come from Seatwave, CEO Joe Cohen said: "We are aware of two instances where Seatwave customers were turned away from the gig.
"In line with our ticket integrity policy we immediately refunded these fans in full."
The band stated on their website that they had been "alerted to the fact that some people have fraudulently been making and selling fake tickets for [the gig]".
"We sincerely hope that not too many of you good, honest Mumford & Sons fans have been affected," they continued. "Remember to always purchase tickets from official vendors only."
Watch the video for 'I Will Wait' below: