La2Zr2O7 is regarded as one potential topcoat material for thermal barrier coatings (TBCs). This study explores doping as a means to further lower the thermal conductivity of La2Zr2O7, and thereby further enhance its candidacy as a topcoat material. Combining density functional theory (DFT) calculations with thermodynamic modeling, we computed the concentrations of various lattice defects, including substitutional defects, oxygen vacancy and interstitial oxygen for the doped system. Based on the defect concentrations, the resultant lattice constants of the doped systems were computed. Further, using the single-mode relaxation-time approximation from first-principles anharmonic lattice dynamics calculations, the lattice thermal conductivity of the doped system can be predicted and were shown to be in very good agreement with experimental measurements. This study presents a complete first-principles density functional theory (DFT) route that enables quantitative prediction of the reduction in thermal conductivity induced by doping, providing a valuable tool towards computational design of TBCs.