High-entropy ceramics (HECs) are promising materials for advanced applications; however, the effects of rare-earth doping on their structural and mechanical properties remain underexplored. Using density functional theory, this study investigates the effects of Er doping on the structural, mechanical, electronic, and dislocation properties of (Ti_0.25Ta_0.25Hf_0.25Nb_0.25-xEr_x)₂Zr₂O₇ (x = 0%–25%) HECs. The disordered ceramic supercell is modeled as a 2 × 2 × 2 special quasirandom structure. The influencing mechanisms of Er doping are elucidated by calculating the critical parameters (formation enthalpy, lattice distortion, elastic constants, mechanical properties, and density of states). Er substitution at the Nb sites minimizes the formation enthalpy, i.e., optimizes the structural stability. Increasing the Er content intensifies the lattice distortion, as evidenced by increased atomic size differences and root-mean-square atomic displacements. The (Ti_0.25Ta_0.25Hf_0.25Nb_0.1875Er_0.0625)₂Zr₂O₇ (x = 6.25%) and (Ti_0.25Ta_0.25Hf_0.25Nb_0.0625Er_0.1875)₂Zr₂O₇ (x = 18.75%) HECs exhibit the highest bulk modulus, Young’s modulus, shear modulus, fracture toughness, and dislocation energy factors. The ductility decreases with an increase in Er content. Moreover, the isotropy increases and anisotropy decreases with an increase in Er content up to 18.75%. Er doping elevates the density of states at the Fermi level, enhancing the metallic and electrical conductivity properties of the HECs. The energy factors are lower at screw dislocations than at edge dislocations, aiding the nucleation of screw dislocations. The edge-dislocation width peaks at 18.75% Er, promoting twinning deformation and improving the plasticity.