This study presents a breakthrough approach to enhance the thermal stability, mechanical properties, and flame retardancy of flexible PVC (fPVC). It introduces lanthanide phytate montmorillonite (Pa-La-Mt) as an innovative nanofiller. Pa-La-Mt was synthesized by intercalating sodium phytate and lanthanum chloride into the interlayer space of montmorillonite (Mt), effectively increasing the layer spacing. The analysis of this study, using X-ray diffraction (XRD) and Fourier infrared spectrometer (FTIR), confirmed this structural transformation. The influence of Pa-La-Mt on the structural and performance aspects of fPVC composites was thoroughly examined. Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), mechanical testing, limit oxygen index (LOI), and cone calorimeter (CONE) revealed compelling outcomes. Pa-La-Mt not only improved the dispersion and compatibility of Mt. within the fPVC matrix but also enhanced the thermal stability and mechanical performance of fPVC composites. Most notably, Pa-La-Mt exhibited remarkable flame retardant and smoke suppression effects on fPVC composites. It led to a substantial increase in LOI value, a reduction in the peak heat release rate and total heat release amount, and a decrease in CO and CO2 emissions. This underlines Pa-La-Mt as a promising nanofiller with profound implications for various fPVC applications. •We present Pa-La-MMT, a green flame retardant from nature, making fPVC safer in an eco-friendly way.•Adding Pa-La-MMT makes fPVC more heat-resistant and stronger, closing the gap between fire safety and structure.•Pa-La-MMT excels as a flame retardant, raising LOI significantly and earning a top V-0 rating in UL-94 tests.