This paper examines the effects of actuator dynamics on the performance of a hydraulic semi-active tuned mass damper. The original semi-active tuned mass damper design proposed by Hrovat (1983) is recounted. A consistent model for the actuator dynamics is established. A practical bistate control strategy is then developed for the system. The performance of the SATMD is obtained using numerical simulations and compared to Hrovat’s results. While the work here suggests that a carefully designed semi-active tuned mass damper system can provide structural vibration mitigation for wind loads, seismic loads can saturate the tuned mass damper system.