H∞ output feedback control strategy for linear systems with time-domain hard constraints was proposed, and its application to active suspensions was presented. In the framework of linear matrix inequality optimization, sufficient conditions for the existence of such a constrained H∞ output feedback controller, as well as detail steps for the controller designing were given. The proposed approach was applied to design active suspension systems on the basis of a two-degree-of-freedom quarter car model, with vertical body acceleration chosen as controlled outputs and dynamic suspension deflection, relative dynamic tire load, and control force chosen as constrained outputs. Simulation results show that the proposed constrained H∞ controller can achieve a promising improvement on ride comfort, while keeping dynamic suspension deflections, dynamic tire loads and control forces within given bounds, even variations of the vehicle model parameters exist.