This study aims to enhance the flame retardant efficiency and thermal stability of low density polyethylene (LDPE) by using halogen-free inorganic additives. To prepare the highly loaded flame retardant polymer composites, LDPE melt mixes with magnesium hydroxide and magnesium carbonate at different weight ratios using a twin-screw extruder. The composites are characterized by thermogravimetric analysis, crystallinity degree, tensile strength, morphological analysis, limiting oxygen index and UL-94 vertical burning test. An optimum MH/MC weight ratio of 60/40 provides satisfactory composite properties with increased maximum thermal degradation temperature from 472.3 to 483.5°C, the highest LOI value of 32.5% and a UL-94 V-0 rating without melt dripping. The homogeneity of this composite is improved by using a polymeric compatibilizer, MAH-g-PE and an enhancement in tensile strength, elongation at break and elastic modulus values of 33.68%, 33.02% and 23.55%, respectively, are obtained. In order to create synergism between the flame retardant additives, 1, 3 and 5 wt% zinc borate is added to the composite system. The incorporation of 5 wt% ZB causes a significant delay in the thermal decomposition of MH with an improvement of 51.8°C in maximum degradation temperature. This composite also exhibits a satisfactory flame retardant grade with a UL-94 V-0 rating.