Bioinspired solar-chemical energy conversion has long been challenged by chemists with searching for effective synthetic photocatalysts. Herein we show that powered by visible light, molecular photocatalysts developed by integrating zinc porphyrin (ZnP) with a quadruply-bonded Mo₂ unit enables hydrogen evolution from water in high yields up to 200?mmol?g^?1 with TON?=?640. Remarkably, this unimolecular Mo₂-ZnP device catalyzes efficiently photoreduction of nicotinamide adenine dinucleotide (NAD⁺) to NADH, the reducing equivalent for enzymatic reduction of CO₂ in natural photosynthesis. Evidently, this Mo₂-ZnP photosystem has the capability of accumulating up to four electrons and two protons for two-electron reduction. Mechanistic investigations reveal that the photoreduced zinc phlorin and chlorin-phlorin anions are the catalytic intermediates for hydride transfer to the substrates, which mimics functionally the hydroquinone state of flavin adenine dinucleotide in photosystem I. The understandings gained in this study are valuable for engineering photocatalysts for solar fuel production.