The group had originally failed to make the show, finished third in their semi-final last Sunday.
But the judges decided to bring the boy group back tonight as the wild card.
After their performance of Usher's Without You, the panel felt justified by their decision.
David Wallaims said: "You've just proved why you were worthy of the wild card. It sounded great, and there was a great energy between you all."
Alesha Dixon added: "I'm so happy for you, there's lots of support out there for you, you look great and you had great vocals. I hope to see you next year at Wembley Area or Stadium!"
Simon Cowell commentted: "Interesting choice of song, little bit shaky at the beginning which I think was down to nerves an then you hit another gear.
"What I like about you guys is that you're current, and anything can happen tonight."
Watch The Mend performing on Britain's Got Talent below...