Design and exploitation of flame retardant polymers with high electrical conductivity are desired for polymer applications in electronics. Herein, a novel phosphorus-nitrogen intumescent flame retardant was synthesized from pentaerythritol octahydrogen tetraphosphate, phenylphosphonyl dichloride, and aniline. Low-density polyethylene was combined with the flame retardant and multi-walled carbon nanotubes to form a nanocomposite material via a ball-milling and hot-pressing method. The electrical conductivity, mechanical properties, thermal performance, and flame retardancy of the composites were investigated using a four-point probe instrument, universal tensile machine, thermogravimetric analysis, and cone calorimeter tests, respectively. It was found that the addition of multi-walled carbon nanotubes can significantly improve the electrical conductivity and mechanical properties of the low-density polyethylene composites. Furthermore, the combination of multi-walled carbon nanotubes and phosphorus–nitrogen flame retardant remarkably enhances the flame retardancy of matrixes with an observed decrease of the peak heat release rate and total heat release of 49.8% and 51.9%, respectively. This study provides a new and effective methodology to substantially enhance the electrical conductivity and flame retardancy of polymers with an attractive prospect for polymer applications in electrical equipment.