The enzyme, characterized from multiple bacterial species, catalyses the hydrolysis of terminal, non-reducing D-mannose residues from β-1,2-mannotriose and β-1,2-mannobiose. The mechanism involves anomeric inversion, resulting in the release of α-D-mannopyranose. Activity with β-1,2-mannotriose or higher oligosaccharides is higher than that with β-1,2-mannobiose.