The concurrent transformation of carbon dioxide and water into hydrocarbons and oxygen by low-photonic-energy IR light still represents a huge challenge. Here, we design an ultrathin conductor system, in which the special partially occupied band serves as the mediator to simultaneously guarantee IR light harvesting and satisfy band edge positions, while the ultrathin configuration improves charge separation rates and surface redox kinetics. Taking the low cost and earth-abundant CuS as an example, we first fabricate ultrathin CuS layers, where temperature-dependent resistivities, valence-band spectra, and theoretical calculations affirm their metallic nature. Synchrotron-radiation photoelectron and ultraviolet visible--near-infrared spectra show that metallic CuS atomic layers could realize a new cooperative intraband interband transition under IR light irradiation, where the generated electrons and holes could simultaneously involve the carbon dioxide reduction and water oxidation reactions. As a result, CuS atomic layers exhibit nearly 100% selective CO production with an evolution rate of 14.5 mu mol g(-1) h(-1) under IR light irradiation, while the catalytic performance shows no obvious decay after a 96 h test. Briefly, benefiting from ultrahigh conductivity and a unique partially occupied band, abundant conductor materials such as conducting metal sulfides and metal nitrides hold great promise for applications as effective IR light responsive photocatalysts.