An alternative design method of sliding mode control for practical implementation on nonlinear isolated bridges is proposed. The proposed controller explicitly reveals the robustness of sliding mode control to parametric uncertainties of structures. The effectiveness of sliding mode control is studied numerically and systematically for an isolated viaduct under extreme earthquakes. The results show that the sliding mode control with appropriate sliding surfaces achieves outstanding control performance compared to the conventional LQR control and viscous damping passive control. Additionally, the performance of active control can be considerably fulfilled by the semiactive control with variable dampers using the proposed control algorithm.