Poly (vinyl chloride) (PVC) has a very high tendency for increasing thermal dehydrochlorination that generates hydrogen chloride (HCl), induces conjugated polyene forming, leads to discoloration at all the same time pushing gradual reduction of mechanical performance. The plant-based polyphenol, ellagic acid (EA), was tested as a bio renewable thermal stabilizer at 0.5, 1, 2 and 3 phr. Measurements encompass articles with experimental test-factors, while kinetic information is a data-supported study. Experimental results showed that 2 phr of EA was the optimal loading in the studied range provided. The apparent first-order dehydrochlorination rate constant decreased from 0.0345 to 0.0078 min−1, and the apparent activation energy increased from 42.3 to 84.7 kJ mol−1 at this concentration. The 5% mass-loss onset temperature increased from 249 to 304 °C, and the DTG maximum shifted from 302 to 355 °C. After 60 min at 180 °C, the yellowness index decreased from 76.8 ± 3.1 to 16.8 ± 0.9, whereas tensile strength increased from 36.2 ± 1.8 to 66.7 ± 2.8 MPa and elongation at break increased from 112 ± 6% to 248 ± 12%. SEM showed fewer cracks and improved surface continuity, while FTIR was consistent with reduced structural deterioration. The proposed radical-scavenging and HCl-interaction effects are supported by the experimentally obtained kinetic, thermal, spectroscopic, colorimetric, and mechanical data. The results support a concentration-dependent stabilization window with an optimum at 2 phr EA.