A reflective element assembly for a vehicular mirror assembly includes a front substrate, a rear substrate, an electro-optic medium disposed between the front and rear substrates and a transmission-reducing thin film coating established at the fourth surface of the rear substrate. A window is established through the transmission-reducing thin film coating and is substantially devoid of the thin film coating at a location where a sensor is disposed behind the reflective element and having a field of view through the reflective element and through the window. A portion of the transmission-reducing thin film coating at and around the window locally varies in physical thickness, with a minimum physical thickness of the thin film coating being closest to the window and with the physical thickness of the thin film coating generally increasing to a generally maximum physical thickness of the thin film coating at a distance from the window.