This study investigated the crystallization and electrical properties of atomic layer deposited Zn–Sn–O (ZTO) thin films. Also, the optimum composition ratio that can provide thermal stability and good device performance for the ZTO thin film through high-temperature annealing was examined. As a result, it was confirmed that the highest crystallization temperature was ∼700 °C in Sn 42–66 at%, and the best device performance was observed at Sn 42 at%. This conclusion is attributed to the difference in the shallow donor-level energy distribution of the Sn-related and the Zn-related oxygen vacancies. Furthermore, the higher the annealing temperature below the crystallization temperature, the better the mobility, presumed to be caused by decreased defect density. However, a sudden decrease in device performance was observed in the region above the crystallization temperature due to the formation of grain boundaries and related defects. Therefore, the optimal Sn concentration to provide thermal stability and good device performance of the ZTO thin film was approximately 42 at%, where promising device performances were achieved, including a carrier mobility of 25.5 cm² V⁻¹ s⁻¹, a subthreshold swing of 0.30 V decade⁻¹, and a threshold voltage of −0.52 V.