A large-aperture, reactionless Fast Steering Mirror (FSM) is described and modeled, and performance data are presented. The FSM optical element is 24 cm in diameter and is capable of +/- 3 mrad angular stroke in both azimuth and elevation. Dedicated position sensors provide feedback to enable two-axis closed-loop operation at frequencies greater than 1 kHz. The system is modeled as a set of coupled differential equations, from which performance predictions can be made. The performance of the system is presented in terms of spectral power consumption, spectral reaction attenuation, and closed-loop behavior. Closed-loop performance is characterized in terms of open-loop, closed-loop, and error attenuation transfer functions. Comparison to predictions from the dynamic model are made where applicable.