Leith picked up the account in 2014 but has not produced any work.
C&C Group restructured its business in the same year, aligning Magners, the Shepton Mallet Cider Mill brands and International Wine Services under C&C Brands.
Fold7 has been tasked with "reimagining the brand by developing and executing a new creative strategy".
Andy Cross, the brand director at Magners, said: "In an increasingly competitive market, we need an agency that can demonstrate first-class strategic thinking and creative ideas.
"We have been impressed with how Fold7 have embraced the challenge as they prove to be the ideal partner to help us achieve our ambitions for Magners."
The Red Brick Road was Magners’ retained agency until 2014. The shop attributed the loss to the brand reducing its adspend.
Marc Nohr, the chief executive of Fold7, said: "Our ambition is to create a real step change for the brand, taking it to the next phase of its development."