The molecular‐level structuration of two full photosystems into conjugated porous organic polymers is reported. The strategy of heterogenization gives rise to photosystems which are still fully active after 4 days of continuous illumination. Those materials catalyze the carbon dioxide photoreduction driven by visible light to produce up to three grams of formate per gram of catalyst. The covalent tethering of the two active sites into a single framework is shown to play a key role in the visible light activation of the catalyst. The unprecedented long‐term efficiency arises from an optimal photoinduced electron transfer from the light harvesting moiety to the catalytic site as anticipated by quantum mechanical calculations and evidenced by in situ ultrafast time‐resolved spectroscopy. Porous organic polymers were used as photosystems to deliver a constant production rate for the CO2 to formate reduction for several days. Their photoactivation pathway is presented, including an ultrafast electronic energy transfer from the photosensitizer to the catalyst as evidenced by time‐resolved spectroscopy and quantum mechanical calculations.