An interior rearview mirror assembly for a vehicle comprises an electro-optic mirror unit whose reflectivity is variable in response to an electrical voltage applied thereto. The mirror unit includes a first substrate and a second substrate with an electro-optic medium sandwiched therebetween. The second substrate includes a recess at a perimeter region thereof. The mirror assembly includes at least one light sensor disposed behind the first substrate of the mirror unit and generally at the recessed perimeter region of the second substrate. The at least one light sensor is operable to receive light through the first substrate and unobstructed by the second substrate of the mirror unit. The light may be received through a bezel of the mirror assembly and through the first substrate and unobstructed by the second substrate.