The influence of Cd doping on the performance of ZnSn(OH)₆ (ZHS) as a gas sensor was systematically investigated through experimental and theoretical approaches. ZHS and Cd-doped ZHS samples were synthesized using the hydrothermal method. The microstructures of pure and Cd-doped ZHS were characterized using various techniques. The results revealed that the pure ZHS sample exhibits good crystallinity and an octahedral morphology with particle sizes ranging from 800 to 1900 nm. After Cd doping, the particle size range was decreased to 700–1500 nm. A systematic investigation of the gas-sensing properties revealed that Cd-doped ZHS exhibits superior sensing performance toward ethanol gas compared to pure ZHS. Under operating conditions of 240 °C, 100 ppm concentration, and 30% relative humidity, the response of ZHS to ethanol gas exhibited a significantly higher value compared to other tested gases. After Cd doping, the response approximately doubled. Density functional theory calculations of electronic structures revealed that the enhanced ethanol sensing mechanism of Cd-doped ZHS is attributed to the narrowed band gap caused by Cd doping, which increases electron concentration and enhances ${\mathsf{O}}^{-}$ ion adsorption on the surface.