Mixed A-cation halide perovskites have emerged as one of the most promising materials for next-generation optoelectronic applications due to such factors as attractive charge carrier transport properties and enhanced stability under operating conditions. However, the influence of A-cation mixing on the excited state charge carrier dynamics and, particularly, on ultrafast hot-carrier relaxation processes, is yet to be studied in sufficient detail. We combine nonadiabatic molecular dynamics and time-domain density functional theory methods to establish the impact of formamidinium (FA)-cesium (Cs) mixing on the subpicosecond-scale hot-charge carrier cooling processes in FA1−xCsxPbI3 (x ≤ 0.5) materials. Our ab initio study illustrates that the partial substitution of organic FA species with inorganic Cs cations substantially extends the hot-electron and hot-hole relaxation times. Observed increases in the hot-carrier lifetimes indicate better performance of FA1−xCsxPbI3 compared to parent FAPbI3 in the field of hot carrier solar cells. The atomistic details of lattice dynamics reveal that FA-Cs cation mixing partially suppresses thermal fluctuations in the structure, weakening the carrier-phonon interaction under ambient conditions. Increased structural rigidity and weakened carrier-phonon interactions in turn lower the rates of intraband nonadiabatic transitions of hot-carriers and enhance their excited state lifetimes. The in-depth understanding of the relationship between the dynamic structure and carrier relaxation allows us to further propose rational design principles that can enhance the hot-carrier lifetimes in photoactive materials. The computational guides will help to realize photovoltaic devices that efficiently harvest hot-carriers and exhibit an improved power conversion performance compared to traditional single-junction solar cells.