Radar resonance-enhanced multi-photon ionization (Radar REMPI) has developed as a diagnostic technique over the past decade, though the effects of an external magnetic field have yet to be considered. A model for Radar REMPI in the presence of a magnetic field is developed and tested in experiment, resulting in a depolarization of the microwave scattering. The effect is found to depend on both the magnitude and direction of the magnetic field, which can lead to over-or under-estimates of Radar REMPI measurements if not taken into account, but also shows the potential for the diagnostic to measure vector magnetic fields. The determined dependence of the Radar REMPI signal on the external magnetic field can be used to improve the measurements of electron and neutral number densities, electron loss rates, and other measurement capabilities of Radar REMPI. Suggestions are made on how to minimize and account for this effect in experiments for other desired measurement quantities.