In this paper, the reliability assessment of magnetorheological (MR) dampers in reducing structures’ seismic responses is studied. Two passive and semi-active control scenarios are considered and discussed. For semi-active control scenario, the Lyapunov control algorithm is used. In order to minimize the structure’s responses under a given earthquake excitation, an unconstrained optimization problem is first defined and solved via genetic algorithm to find an optimal diagonal matrix in Lyapunov’s control algorithm. The procedure is explained for a three-storey benchmark shear structure equipped with MR damper in all storeys. The structure’s responses reliability for various uncertainties are derived and interpreted based on standard Monte Carlo simulation approach. In the reliability problem, for the limit state function two cases based on human comfort and structural safety criteria are defined and investigated. Furthermore, the most influential uncertain variables and control scenarios on structures’ reliability are identified. Finally, a sensitivity analysis on the coefficient of variations of random variables on the structure’s responses reliability is accomplished. The results show that in the joint event, the MR damper is more reliable for acceleration control rather than drift control in passive control scenario. The aforementioned trend is reversed for semi-active control scenario. Additionally, passive control scenario is more reliable than the semi-active control scenario in reducing maximum acceleration of structure. However, in reducing maximum drift of structure, the values of the reliability in semi-active control scenario are almost similar to passive control scenario.