This study aims to improve the experimentally low performance of p-SnS/n-ZnMgO thin film solar cells (TFSCs). We report a modification in the p-SnS/n-ZnMgO cell structure to address the issues with the help of detailed numerical modeling and analysis via solar cell capacitance simulator software (SCAPS). Here, CdS is used as a thin buffer layer about a few nanometers in between the p-SnS absorber layer and n-ZnMgO window layer. However, in terms of band alignment, SnS/CdS interface attributed the minimum band-offset, resulting in the enhancement of open-circuit voltage (Voc) and overall performance. Furthermore, to evaluate the final cell structure, the solar cell simulation has been investigated by varying several parameters such as thickness and defect density of absorber layer; interface defect density and the operating temperature affect the electrical parameters of TFSCs. Initially, the band-alignment engineering has been investigated for variable doping concentration (x) of magnesium (Mg) in the Zn1-xMgxO window layer. However, Mg concentration (x) = 0.18 shows the better results (Voc = ~ 0.7 V, short-circuit current density (Jsc) = 38.54 mA/cm2, Fill Factor = 83%, and efficiency (ɳ) = ~ 23%) with minimum band-offset at the CdS/ZnMgO interface, and the hexagonal nanorod-like morphology of ZnMgO helps to improve open-circuit voltage. Finally, with the optimized parameters (tSnS = 2 μm, tCdS = 50 nm, and tZnMgO = 70 nm) with maximum SnS/CdS interface defect density (Nt = 1 × 1011 cm−2), the simulated optimal p-SnS/CdS/n-ZnMgO cell structure exhibited the highest efficiency ~ 20% comparably higher than the reported p-SnS/n-ZnMgO experimental value of 2.1%.