This brief presents a novel control method for half-vehicle active suspension systems driven by hydraulic actuators. First, a coordinate transform approach is introduced to reformulate the strict-feedback system into a canonical form, by which the widely used backstepping method is not required. Then, a modified high-gain observer (HGO) is studied to rebuild the unknown system states of the nonlinear active suspension system. To eliminate the effect of unknown nonlinearities, a simple robust unknown system dynamics estimator (USDE) with only one tuning parameter is developed in terms of the system input and output only. Finally, the observer and estimator are integrated to design an output feedback controller to regulate the vehicle motion. Comparative experiments are carried out to illustrate the effectiveness of the proposed method.