A mirror reflective element assembly for an exterior rearview mirror assembly includes a mirror casing, a reflective element, an auxiliary wide angle element and a back plate. The auxiliary wide angle element is disposed at the rear surface of a mirror substrate. A back plate mounting structure may establish a space at the mirror actuator that is sufficient for a portion of the auxiliary wide angle element to be disposed between a rear surface of the reflective element and the mirror actuator. The back plate may have at least one sealing element at least partially around a perimeter region of a wide angle element receiving portion of the back plate, and the sealing element may engage the rear surface of the mirror substrate when the back plate is attached to the reflective element and may limit intrusion of adhesive between the back plate and the mirror substrate.