Hybrid organic–inorganic lead halide perovskite nanoparticles are promising candidates for optoelectronic applications. This investigation describes the structural and optical properties of MA_xCs_1−xPbBr₃ mixed cation colloidal nanoparticles spanning the complete compositional range of Cs substitution. A monotonic progression in the cubic lattice parameter (a) with changes in the Cs⁺ content confirmed the formation of mixed cation materials. More importantly, time-resolved photoluminescence (TRPL) revealed the optimized 13 mol% Cs nanoparticle composition exhibits the longest charge carrier lifetime and enhancement in radiative pathways. This sample also showed the highest photoluminescence quantum yield (PLQY) of ∼88% and displays ∼100% improvement in the PLQY of pure MAPbBr₃ and CsPbBr₃. Prototype LEDs fabricated from MA_0.87Cs_0.13PbBr₃ were demonstrated.