The radiation shielding characteristics of the polyester resin composites reinforced with cerium(iv) oxide (CeO2) have been studied. The prepared composites were pure polyester–resin (Poly/CeO2-0), 90% per weight polyester resin and 10% CeO2 (Poly/CeO2-10), (Poly/CeO2-30), (Poly/CeO2-50), and (Poly/CeO2-60). The linear attenuation coefficient (LAC) values for the free polyester and polyester samples with CeO2 were experimentally measured compared with the XCOM data. The experimental LAC value was found to be 0.2377 cm−1 at 0.0595 MeV, which is in good agreement with the calculated value of 0.2454 cm−1. Also, for the same sample, the experimental LAC was found to be 0.1034 cm−1 at 0.662 MeV, showing a good agreement with the calculated value of 0.1057 cm−1. The LAC values for the free polyester, Pol/CeO2-30, and Pol/CeO2-60 are 1.43, 31.82, and 107.77 cm−1 at 0.015 MeV, respectively. The big difference in the LAC values between the composite with 0 and 60% CeO2 is evident. The radiation shielding efficiency (RSE) of the polyester with different amounts of CeO2 was experimentally measured at four energy values. Also, we extended the calculation of RSE at other energy values in the range of 0.015–15 MeV). The exposure buildup factor (EBF) values for the free CeO2 sample and the samples with CeO2 are calculated. The EBF is small at low energies, then increases, and attains a maximum value at moderate energy; the EBF shows a decreasing trend with an increase in the energy.