This study investigates an extended state observer (ESO)-based sliding mode control (SMC) approach for pulse-width modulation-based DC-DC buck converter systems subject to mismatched disturbances. By designing a novel sliding-mode manifold incorporated with a disturbance estimation technique, an ESO-based SMC method is designed to deal with mismatched disturbances. A rigorous stability analysis is also presented. As compared with the nominal SMC method, the proposed method obtains a better disturbance rejection ability even the disturbances do not satisfy the so-called matching condition. Simulations and experimental comparison results are implemented to verify the effectiveness of the proposed control method.