Recently, it has been demonstrated that control techniques based on sliding mode control (SMC) are robust and their performances are quite remarkable for applications to active/hybrid control of seismic-excited linear, non-linear or hysteretic civil engineering structures. In this paper, sliding mode control methods are further extended by introducing a compensator. The incorporation of a compensator provides (i) a convenient way of making trade-offs between control efforts and specific response quantities of the structure through the use of linear quadratic optimal control theory, and (ii) a convenient design procedure for static output feedback controllers to facilitate practical implementations of control systems. Since civil engineering structures generally involve excessive degrees of freedom, a controller design based on a full-order system may be difficult, in particular for wind-excited tall buildings. In this paper, three reduced-order control methods have been used and their performances have been investigated. Applications of sliding mode control with compensators to active control of buildings subject to either earthquakes or strong wind gusts have been demonstrated through numerical simulations. Simulation results show that the performance of the sliding mode controller with compensators for the reduced-order system is quite close to that of the controller based on the full-order system as long as enough vibrational modes are taken into account in the reduced-order system.