An exterior rearview mirror system suitable for use on a vehicle includes an exterior rearview mirror assembly, a mirror heating element and a control. The mirror assembly includes a mirror head portion mountable to a side of a vehicle and a reflective element supported at the mirror head portion. The mirror heating element is disposed at a rear side of the reflective element and operable to heat the reflective element. The control is operable to power the mirror heating element to heat the reflective element. The control controls heating of the mirror heating element responsive at least in part to a signal indicative of air flow at or around the exterior rearview mirror assembly and the reflective element of the equipped vehicle.