The off-stoichiometry effects and gram-scale production of luminescent CuInS2-based semiconductor nanocrystals, as well as their application in electroluminescence devices are reported. The crystal structures and optical properties of CuInS2 nanocrystals can be significantly influenced by controlling their [Cu]/[In] molar ratio. A simple model adapted from the bulk materials is proposed to explain their off-stoichiometry effects. Highly emissive and color-tunable CuInS2-based NCs are prepared by a combination of [Cu]/[In] molar ratio optimization, ZnS shell coating, and CuInS2–ZnS alloying. The method is simple, hassle-free, and easily scalable to fabricate tens of grams of nanocrystal powders with photoluminescence quantum yields up to around 65%. Furthermore, the performance of high-quality CuInS2-based NCs in electroluminescence devices is examined. These devices have lower turn-on voltages of around 5 V, brighter luminance up to approximately 2100 cd m−2 and improved injection efficiency of around 0.3 lm W−1 (at 100 cd m−2) in comparison to recent reports.