Lanthanum ferrite (LaFeO3) perovskite offers high structural stability and strong visible-light absorption, yet its optoelectronic response is frequently limited by trap-assisted losses and non-radiative recombination. Here, we systematically assess site-specific Mg doping by comparing substitution at La3+ (A-site) and Fe3+ (B-site) in LaFeO3 thin films, and we establish a unified structure–defect–property correlation by combining XRD, FE-SEM, UV–Vis DRS, PL, XPS, dark electrochemical impedance spectroscopy, and DFT(+U) defect formation-energy calculations. Moderate B-site Mg substitution (x = 0.1, LFM01) delivers the most favorable film quality, with compact grains and a controlled oxygen-vacancy population. Power-dependent PL yields β values approaching unity (β ≈ 1.0), narrower emission linewidths, and negative exciton binding energies, consistent with a defect-optimized regime where recombination becomes more delocalized and band-to-band–like. In contrast, A-site Mg doping (LMF01) exhibits a higher β exponent (β ≈ 1.3) together with positive exciton binding energies and broadened, strongly quenched PL, indicating recombination dominated by deep, strongly localized non-radiative traps. At higher B-site loading (LFM03), vacancy clustering drives a defect-saturated regime marked by reduced PL efficiency, broader emission, and β < 1 (β ≈ 0.9). Dark EIS identifies LFM01 as the most conductive film (lowest transport resistance), while DFT confirms Fe-site Mg is thermodynamically favored and lowers the oxygen-vacancy formation energy, rationalizing the coupled defect, photophysical, and transport trends. These findings highlight the critical role of dopant site selectivity in modulating defect states and recombination dynamics, offering valuable insights for optimizing LaFeO3-based materials for future optoelectronic technologies. © 2026 Elsevier Ltd