Global plastic production has reached 7.8 billion tonnes cumulatively since 1950, which has accounted for a myriad of ecological and health problems since most plastics are not biodegradable. The existing waste management solutions for plastic wastes such as landfill disposal and incineration for energy recovery are not environmentally friendly. Moreover, not all plastic wastes are recyclable (e.g., vulcanized rubber and cured thermoset resin). This research aimed to enhance electrical properties of conductive polystyrene (PS)/carbon nanotube (CNT) nanocomposites by incorporation of a non-recyclable plastic waste consisting of mainly vulcanized rubber (60%) and polyvinyl chloride (35%). The waste material is a plastic foam used in sporting goods, and due to high content of vulcanized rubber, it does not melt or dissolve in a solvent. The non-melting property of the plastic waste induced excluded volume effect during melt mixing of PS/Waste/CNT nanocomposites and led to double percolation morphology with enhanced conductive network formation. PS/Waste/CNT samples showed significant improvement in electrical conductivity (i.e., almost 4 orders of magnitude) and electromagnetic interference shielding performance compared to PS/CNT. This upcycling strategy in which a non-recyclable plastic waste was used as an additive to enhance final properties can be employed to develop cost-effective products for a wide range application and address environmental concerns simultaneously.