Catalyzes the dehydrogenation of (S)-mandelate to phenylglyoxylate (benzoylformate) (PubMed:10493804, PubMed:19465768). Is likely involved in the utilization of mandelate as a sole source of carbon and energy for growth (Probable). Active in vitro with the artificial electron acceptors 2,6-dichlorophenolindophenol (DCPIP) or ferricyanide, but in vivo most likely transfer the electron pair from the reduced flavin to a component of the electron transport chain in the membrane, possibly a quinone (PubMed:10493804, PubMed:19465768). Shows very low activity with oxygen as the electron acceptor, and also with 3-indolelactate and medium chain 2-hydroxyacids as substrates (PubMed:10493804, PubMed:19465768). (S)-mandelate + A = AH2 + phenylglyoxylate kcat is 290 sec(-1) with (S)-mandelate as substrate and DCPIP as electron acceptor (at pH 7.5 and 20 degrees Celsius) (PubMed:10493804). kcat is 0.5 sec(-1) with 2-hydroxyoctanoate as substrate and DCPIP as electron acceptor (at pH 7.5 and 20 degrees Celsius) (PubMed:10493804, PubMed:19465768). kcat is 0.34 sec(-1) with 2-hydroxyhexanoate as substrate and DCPIP as electron acceptor (at pH 7.5 and 20 degrees Celsius) (PubMed:10493804). kcat is 0.10 sec(-1) with 2-hydroxybutanoate as substrate and DCPIP as electron acceptor (at pH 7.5 and 20 degrees Celsius) (PubMed:10493804). kcat is 1.0 sec(-1) with 3-indolelactate as substrate and DCPIP as electron acceptor (at pH 7.5 and 20 degrees Celsius) (PubMed:10493804, PubMed:19465768). kcat is 350 sec(-1) with (RS)-mandelate as substrate and DCPIP as electron acceptor (at pH 7.5 and 20 degrees Celsius) (PubMed:19465768). Aromatic compound metabolism; (R)-mandelate degradation; benzoate from (R)-mandelate: step 2/4. Homotetramer. Belongs to the FMN-dependent alpha-hydroxy acid dehydrogenase family.