Molybdenum (Mo)-promoted intermetallic platinum-cobalt (Pt-Co) oxygen reduction reaction (ORR) catalyst with significantly enhance ORR activity was obtained through impregnation and thermal treatments using MoCl₅ as a precursor. With an optimized coverage of the Mo promoters, X-ray absorption spectroscopy reveals the cluster-like Mo-O_x species effectively lowered the white line intensity of Pt in its near edge structure (Pt L-edge XANES), suggesting the further weakening of its affinity towards oxygen. The Mo-promoted intermetallic Pt-Co catalyst thus exhibited a high ORR mass activity of 0.89 A/mg_Pt compared to 0.22 A/gm_Pt for pure Pt and 0.62 A/mg_Pt for its parent Pt-Co respectively under RDE testing conditions. The Mo-promoted catalyst showed compelling durability losing only 29 % of its initial 0.95 V mass activity which is even higher than the non-promoted fresh PtCo catalyst (0.32 A/mg_Pt vs 0.24 A/mg_Pt). When tested under membrane electrode assembly (MEA) conditions, the high ORR activity further enabled the operation of the fuel cell under low cathode relative humidities (RH) with minimal penalties in ionic transport resistance at low current regions. Density functional theory calculations reveal the beneficial role of molybdenum oxide cluster species in lowering the d-band energy surface Pt atoms, particularly at Pt segregated regions with no Pt-Co phase in proximity to induce the strain or ligand effects.