This study focuses on the fabrication and characterization of B4C and nano-Er2O3-reinforced Al7075 metal matrix composites, with compositions of Al7075-(10–n)B4C–nEr2O3 (n = 0, 2, 4, 6 wt%). The composites were produced through high-energy ball milling, cold isostatic pressing, and sintering at 550 °C. Phase constitution, microstructure, and mechanical properties were examined before and after Co-60 gamma irradiation at doses of 50, 100, and 200 kGy. X-ray diffraction (XRD) analyses revealed α-Al as the primary phase in all compositions. An increase in Er2O3 content resulted in peak broadening and slight 2θ shifts, indicating enhanced lattice strain, increased defect density, and grain refinement. No intermetallic reaction phases formed between the Al7075 matrix and B4C/Er2O3 reinforcements. SEM/EDS observations showed that the composite containing 4-Er2O3 and 6-B4C exhibited the most homogeneous reinforcement distribution, lowest porosity, and superior sintering integrity, while 6-Er2O3 led to particle agglomeration and microstructural degradation. Vickers microhardness measurements demonstrated that Er2O3 addition and moderate gamma doses (≤100 kGy) enhanced hardness through dispersion and radiation hardening, but a significant decrease in hardness occurred at 200 kGy due to radiation-induced pore and crack formation. Gamma shielding performance was experimentally evaluated using Am-241 (59.5 keV) and Cs-137 (662 keV) sources in narrow-beam geometry and validated by PHITS Monte Carlo simulations. Increasing Er2O3 content improved the linear attenuation coefficient by approximately ∼63% at 59.5 keV and ∼11% at 662 keV, despite a decrease in B4C content. Overall, the 4-Er2O3 composite is a promising lightweight structural gamma-shielding material for aerospace and nuclear applications.