A medical electrical lead having a conductor assembly covered by an insulating layer, and a shield covering positioned adjacent or proximate to at least a portion of the insulating layer in order to shield the conductor assembly from one or more electromagnetic fields. The shield covering is formed of a polymer-matrix composite. The polymer-matrix composite includes a polymeric resin having discontinuous conductive fillers provided therein. The discontinuous conductive fillers include one or more of nano-sized metal structures and nano-sized non-metallic conductive structures. The nano-sized non-metallic conductive structures can have a coating formed of one or more metals. The nano-sized non-metallic conductive structures can be formed of carbon. In turn, the nano-sized non-metallic conductive structures can include one or more of carbon nanofibers, carbon filaments, carbon nanotubes, and carbon nanoflakes.