A vehicular cabin monitoring system includes a vehicular interior rearview mirror assembly configured for mounting at an interior portion of an equipped vehicle and having a mirror head accommodating an electrochromic mirror reflective element, a driver monitoring camera that is operable to capture image data, and a near infrared light emitter that, when electrically operated to emit near infrared light, illuminates at least the head of the driver. With the vehicular interior rearview mirror assembly mounted at the interior portion of the equipped vehicle, and as the equipped vehicle travels along a road, image data captured by the driver monitoring camera is processed at an electronic control unit to detect light emanating from at least one headlamp of at least one trailing vehicle traveling along the road behind the equipped vehicle.