We have achieved high-efficiency polycrystalline perovskite light-emitting diodes (PeLEDs) based on formamidinium (FA) and cesium (Cs) mixed cations without quantum dot synthesis. Uniform single-phase FA(1-x)Cs(x)PbBr(3) polycrystalline films were fabricated by one-step formation with various FA:Cs molar proportions; then the influences of chemical composition on film morphology, crystal structure, photoluminescence (PL), and electroluminescence (EL) were systematically investigated. Incorporation of Cs+ cations in FAPbBr(3) significantly reduced the average grain size (to 199 nm for FA:Cs = 90:10) and trap density; these changes consequently increased PL quantum efficiency (PLQE) and PL lifetime of FA(1-x)Cs(x)PbBr(3) films and current efficiency (CE) of PeLEDs. Further increase in Cs molar proportion from 10 mol % decreased crystallinity and purity, increased trap density, and correspondingly decreased PLQE, PL lifetime, and CE. Incorporation of Cs also increased photostability of FA(1-x)Cs(x)PbBr(3) films, possibly due to suppressed formation of light-induced metastable states. FA(1-x)Cs(x)PbBr(3) PeLEDs show the maximum CE = 14.5 cd A(-1) at FA:Cs = 90:10 with very narrow EL spectral width (21-24 nm); this is the highest CE among FA-Cs-based PeLEDs reported to date. This work provides an understanding of the influences of Cs incorporation on the chemical, structural, and luminescent properties of FAPbBr(3) polycrystalline films and a breakthrough to increase the efficiency of FA(1-x)Cs(x)PbBr(3) PeLEDs.