Seismic events leading to catastrophic outcomes around the world, particularly in built-up regions surrounding fault lines, often have high death tolls and cause costly damages to societies' existing infrastructure. To suppress damaging vibrations in multi-story buildings across a wide frequency spectrum, much research has been put into the study of variable-resonance tuned mass dampers, which maintain their usefulness across a range of frequencies, unlike passive alternatives. As a novel implementation of fast responsive magnetorheological materials to enable variable resonance, this paper presents a prototype magnetorheological-fluid-based pendulum tuned mass damper, integrating a differential gearbox to yield a damper-controlled transmission between the pendulum mass and a mechanical spring. The device is demonstrated to be highly effective, at its best reducing peak relative displacement by 12.8%, and peak acceleration by 22.0%, in contrast to comparable passive tuning modes in scale-building seismic experiments. This is owed to its controllable resonance which can be increased by 104% from its base value at 2.24 Hz. Further, other performance benefits are demonstrated in RMS structure displacement, and interstory drift ratio.