A systematic evaluation of enhancing photocatalysis via aliovalent cation doping is conducted. Cation In3+, being p-type-doped, was chosen to substitute the Sn site (Sn4+) in Li2SnO3, and the photodegradation of 2,4-dichlorophenol was applied as a model reaction. Specifically, Li2Sn0.90In0.10O3 exhibited superior catalytic performance; the photodegradation efficiency reached about 100% within only 12 min. This efficiency is far greater than that of pure Li2SnO3 under identical conditions. Density functional theory calculations reveal that introducing In3+ increased the electron mobility, yet decreased the hole mobility, leading to photogenerated carrier separation. However, photoluminescence and time-resolved photoluminescence suggest that In3+ induced nonradiative coupling in the matrix, reducing the photogenerated carrier separation ratio compared with that of Li2SnO3. The optical band gap of Li2Sn0.90In0.10O3 was almost unchanged compared with that of Li2SnO3 via ultraviolet-visible absorption. The increased photocatalytic efficiency was ascribed to the lower valence band position and enhanced hole concentrations by valence band X-ray photoelectron spectroscopy and electrochemical measurements. Finally, a 2,4-dichlorophenol degradation pathway, an intermediate toxicity assessment, and a photocatalytic mechanism were proposed. This work offers insights into designing and optimizing semiconductor photocatalysts with high performance.