This paper introduces a novel sliding mode controller to regulate frequency within a decentralized control framework. The decentralized frequency regulation model is a continuous-time linear system with mismatch disturbance that cannot be effectively removed. The proposed method combines the sliding mode and active disturbance rejection theories to reject the effects of mismatched disturbances with the aid of a disturbance observer to achieve a fully decentralized frequency regulation control. Furthermore, the sliding variables can be directly computed using the proposed steady-state method. In addition, by incorporating a constant delay into the input of the sliding mode controller, the transient performance is improved, resulting in a smaller overshoot and a shorter response time. Specifically, the proposed method demonstrated enhancements over the liner active disturbance rejection algorithm (5.04% and 54.4%, respectively), conventional proportional integral derivative method (5.09% and 68.03%, respectively), disturbance observer–based integral sliding mode control method (6.59% and 84.34%, respectively), and disturbance observer–based adaptive integral sliding mode control method (5.93% and 82.47%) in terms of both overshoot and response time.