Metal hydroxides have long been considered as abundant and low-cost fillers for the development of halogen-free flame-retarded polyolefins. However, large filler loading levels are required in the matrix resin to achieve satisfactory fire retardant performance, which results in deteriorated processing characteristics and poor mechanical properties of compounds. Therefore, this study was aimed at improving the fire retardancy of low-density polyethylene and decreasing the total filler loading in the resin using combinations of aluminum hydroxide and a natural hydrated calcium borate, namely, colemanite mineral. Flame retardancy was studied by the mass loss calorimeter analysis, limiting oxygen index measurements, and UL94 classification. Peak heat release rate and total heat evolved were lowered by around 20% and 15%, respectively, with partial replacement of aluminum hydroxide by colemanite, which facilitated the use of lower total filler loading in the resin. Depending on colemanite content and total filler loading, limiting oxygen index was increased by 1%-4% while maintaining the UL94 V-0 rating. Fire retardant effect of colemanite used in conjunction with aluminum hydroxide was attributed to better protective character of fire residues and more effective fuel trapping in the condensed phase. Lower elastic modulus and higher ductility was obtained due to decreased total filler loading in the matrix.