A mirror reflector sub-assembly for a vehicular exterior rearview mirror assembly includes a mirror reflective element having a glass substrate. A principal reflector portion has a first radius of curvature and an auxiliary reflector portion includes a curved recess having a second radius of curvature smaller than the first radius of curvature. The curved recess is established at the second surface of the glass substrate by physical removal of glass from a portion of the substrate. The mirror reflective element is supported at a side of a mirror back plate and an opposing side of the mirror back plate is adapted for attaching the mirror back plate to a mirror actuator of an exterior rearview mirror assembly equipped with the mirror reflector sub-assembly. A heater pad is disposed at the second surface of the glass substrate between the mirror reflective element and the mirror back plate.