All inorganic CsPbX₃ (X = Cl, Br, and I) perovskite nanocrystals have made unprecedented radical progresses for opto-electronic applications, but their instability and toxicity of lead impede their wide applications. Herein, we report the first precipitation of lead-free Mn:Cs₂ZnX₄ nanocrystals in glasses. Efficient green and red photoluminescence are realized from these Mn:Cs₂ZnX₄ nanocrystals in glasses. By adjusting the size and halide component in these nanocrystals, coordination environment of Mn²⁺ ions can be tuned, leading to tunable photoluminescence with improved quantum efficiency. It is illustrated that in-situ precipitation of these Mn:Cs₂ZnX₄ nanocrystals in glasses significantly improves their thermal-, chemical-, and photo-stabilities, making them promising for light-emitting diodes with stable chromaticity coordinates.