A vehicular rearview mirror system and method of controlling the vehicle rearview mirror system including establishing a most-dimmed reflectance level of the interior mirror reflective element that is higher than a most-dimmed reflectance level of the exterior mirror reflectance level, the establishment being a function of the rear window light transmission value. The establishment may be a function of sampling of light sensed by at least one light sensor in the passenger compartment that is directed toward the rear-viewing rear window to receive light passing through the rearward-viewing rear window.