This study sheds light on the effect of iodide substitution on the structural and optical properties of solution processed mixed cesium lead perovskite thin films CsPbBr_3−xI_x (0 ≤ x ≤ 1) as well as their atmospheric and thermal stability. Pawley fit indicated that the mixed lead halide CsPbBr_3−xI_x perovskite crystallizes in the orthorhombic (Pnma) space group. X-ray diffraction analyses showed a shift to lower angles when the average fraction of iodide incorporated increased indicating the expansion of the lattice. Absorbance measurements show that the CsPbBr_3−xI_x films exhibit a very high absorbance in the visible and short infrared region. The band gap of CsPbBr_3−xI_x is tuned from 2.38 (CsPbBr₃) to 2.17 eV (CsPbBr₂I₁) following the Vegard's law. CsPbBr₃ films showed a bright photoluminescence with emission maximum at 530 nm CsPbBr_3−xI_x films demonstrate excellent stability after aging for two days in a humid environment (relative humidity of ∼60%) and under thermal annealing at relatively high temperature.