Catalyzes the oxidation of (S)-mandelate to benzoylformate and enables utilization of mandelate as substrate for growth. (S)-mandelate + A = AH2 + phenylglyoxylate Binds 1 FMN per subunit. Binds 1 heme b group per subunit. Aromatic compound metabolism; (R)-mandelate degradation; benzoate from (R)-mandelate: step 2/4. Homotetramer. In the N-terminal section; belongs to the cytochrome b5 family. In the C-terminal section; belongs to the FMN-dependent alpha-hydroxy acid dehydrogenase family.