A vehicular video mirror system includes an interior rearview mirror assembly having a transflective reflective element. The mirror system includes a video display device at a casing of the mirror assembly rearward of the transflective reflective element, with the video display device having a video screen and a plurality of individual white light emitting light sources operable for backlighting the video screen. The intensity of light emitted by the white light emitting light sources is variable responsive to detection of light by at least one photosensor. The video screen may be operable to display video images captured by a rear back-up camera of the equipped vehicle during a reversing maneuver of the equipped vehicle. Light emanating from the white light emitting light emitting diodes may pass through a brightness enhancement film and a light diffuser to be incident at a transflective reflector of a second substrate of the mirror assembly.