The current work deals with studying the influence of cheap, widespread CaO on improving the γ ray-protection capacity of the lightweight, sealing polyester. Therefore, polyester composites were filled with different concentrations of CaO fillers. The fabricated CaO-reinforced polyester composites’ density ranged between 1.177 g/cm3 and 1.377 g/cm3, when CaO filler concentrations increased between 0 wt% and 60 wt%, respectively. Fabricated composites’ morphology and chemical composition, as well as CaO fillers’ grain size and distribution, were proved using SEM and EDX spectroscopy. Additionally, the influence of CaO fillers on the gamma-ray shielding properties of the fabricated composites was evaluated using the Monte Carlo simulation and confirmed using the experimental measurements. The recorded results show an enhancement in the synthesized composites’ linear attenuation coefficient from 0.091 cm−1 to 0.106 cm−1 at a gamma ray energy of 0.662 MeV. Moreover, the excess in CaO concentration from 0 wt% and 60 wt% reduces the fabricated composites’ half-value thickness values from 7.64 cm to 6.51 cm, respectively.