We report high-performance Mg-doped ZnO thin film transistors (TFTs) with various parameters fabricated using a sol-gel solution process. An appropriate amount of Mg doping enhanced the electrical performance of the TFTs. The ZnO TFTs doped with 0.5 at% Mg exhibited high performances with average mobilities of 0.61 and 0.64 cm2V-1s-1 when annealed at 400 and 450 °C, respectively, along with high Ion/Ioff ratios on the order of 104. The chemical composition was examined using X-ray photoelectron spectroscopy and it was confirmed that the Mg was doped in the ZnO thin films at both annealing temperatures. The improved electrical performance was attributed to an increase in the grain size at the optimized doping concentration.