<p>Formamidinium-methylammonium mixed-halide (FAMA) perovskites are promising candidates for high-performance and stable perovskite solar cells (PSCs). However, their preparation and stability under ambient conditions are a big challenge for large scale production. Here, we systematically investigate the influence of MAPbBr₃ incorporation on the structural and optoelectronic properties of (FAPbI₃)_1-x(MAPbBr₃)_x films. Crucially, all films and devices are fabricated entirely under ambient conditions with high relative humidity (up to 65%), without the use of solutions additives or inert gas protection. X-ray diffraction analysis reveals that higher MAPbBr₃ content induces lattice contraction, stabilizes the photoactive α-phase, and suppresses the formation of the detrimental δ-phase of FAPbI₃. Correspondingly, optical analysis demonstrates a tunable band gap from 1.58 to 1.88&#xa0;eV as MAPbBr₃ content increases, which is consistent with the changes observed in the Pb-X-Pb bond angles and halide substitution. Parallel to these structural improvements, electronic evaluation via Urbach energy analysis confirms a substantial reduction in sub-bandgap states and crystalline disorder at higher MAPbBr₃ compositions. Due to these improvements, devices fabricated under ambient conditions exhibit enhanced open-circuit voltage and fill factor, reaching a champion efficiency of 13.1% for (FAPbI₃)_0.7(MAPbBr₃)_0.3. Importantly, humidity-dependent tests identify (FAPbI₃)_0.7(MAPbBr₃)_0.3 as the optimal composition, balancing the benefits of δ-phase suppression and crystalline quality against the increased moisture sensitivity from excessive MA⁺. FAMA samples with a stabilized α-phase of FAPbI₃ and a low PbI₂ concentration show improved moisture resistance compared to pure FAPbI₃, suggesting a potential pathway for more stable devices. These findings underscore the potential of compositionally engineered FAMA perovskites for efficient and scalable solar cell fabrication under realistic environmental conditions.</p>