A vehicular exterior rearview mirror assembly includes a mirror reflective element sub-assembly having a mirror reflective element, a mirror reflector, a mirror back plate, and indicator element. The indicator element includes a housing and a light source. The indicator element is associated with a blind spot detection system of the vehicle and includes a diffuser that diffuses light emitted by the light source. An icon is established through a metallic reflector coating disposed at the glass substrate. The icon includes a light-transmitting aperture. When the vehicular exterior rearview mirror assembly is attached at the vehicle, the icon is at an upper quadrant of the mirror reflective element. When the vehicular exterior rearview mirror assembly is attached at the vehicle and when the light source is activated, the icon is illuminated and is viewable by the driver of the vehicle.