A proximity sensor may be mounted below a display cover layer in an electronic device. The proximity sensor may have a light source that emits light and a detector configured to detect reflections of the emitted light from nearby external objects. The light emitted from the light source may pass through a lens along an axis towards external objects. The light source and the detector may be mounted in a proximity sensor housing having openings that are aligned with the light source and the detector. A reflector may be mounted to the proximity sensor in a configuration that bridges the opening over the light source. The reflector may be formed from a strip of metal or a strip of prism structures. Some of the light from the light source reflects from the reflector at a non-zero angle with respect to the axis and enhances proximity sensor performance.