The underside of an inactive portion of a display cover layer in an electronic device may be covered with an opaque masking material. Openings in the opaque masking material may be form ambient light sensor and proximity sensor windows. An ambient light sensor window may be filled with a material that transmits at least some visible light. A proximity sensor window may be filled with a material that transmits more infrared light relative to visible light than the material in the ambient light sensor window. The materials in the ambient light sensor window and proximity sensor window may include one or more layers of ink, patterns of holes, layers of material that are shared with the opaque masking layer, and materials that are black, white, or other colors. A light guide structure may be used to route light received from a sensor window to an associated sensor.