As a promising molecular catalyst for oxygen evolution reaction (OER), metalloporphyrin is a good model system that is extensively studied. The catalytic efficiency of metalloporphyrin can be improved with deeper insight into its complex issues, such as structural stability and catalytic activity. Using in situ electrochemical scanning tunneling microscopy (EC-STM) and X-ray photoelectron spectroscopy, the morphological evolution of the manganese porphyrin/Au(111) interface affected by the electrocatalytic reaction is revealed. In alkaline solution, the catalytic performance is dramatically enhanced after the first potential sweep, directly related to the formation of the MnOx-porphyrin complexes, driven by an irreversible oxidation-reduction process. These newly formed catalytically active materials exhibit synergistic effects with the Au interface. In situ EC-STM imaging provides the molecular evidence for the formation of the real active metalloporphyrin-based catalyst, showing the complicated interrelation of the morphology, structure, catalytic activity, and electrolyte in OER catalysts.