The biological production of FDCA is of considerable value as a potential replacement for petrochemical derived monomers such as terephthalate, used in polyethylene terephthalate (PET) plastics. HmfF belongs to an uncharacterised branch of the prenylated flavin (prFMN) dependent UbiD-family of reversible (de)carboxylases, and is proposed to convert 2,5-furan-dicarboxylic acid (FDCA) to furoic acid in vivo. We present detailed characterisation of HmfF and demonstrate that HmfF can catalyse furoic acid carboxylation at elevated CO2 levels in vitro. We report the crystal structure of a thermophilic HmfF from Pelotomaculum thermopropionicum, revealing the active site located above the prFMN cofactor contains a furoic acid/FDCA binding site composed of residues H296-R304-R331 specific to the HmfF branch of UbiD enzymes. Variants of the latter are compromised in activity, while H296N alters the substrate preference to pyrrol compounds. Solution studies and crystal structure determination of an engineered dimeric form of the enzyme revealed an unexpected key role for a UbiD-family wide conserved Leu residue in activity. The structural insights into substrate and cofactor binding provide a template for further exploitation of HmfF in the production of FDCA plastic precursors, and improve our understanding of catalysis by members of the UbiD enzyme family.