Lead mixed-halide perovskites are promising absorption materials that are suitable for applications in tandem solar cells using existing silicon technology. Charge-carrier mobility is an important factor that affects the performance of tandem solar cells. However, a detailed understanding of the fundamental mechanisms of lead mixed-halide perovskites remains elusive. Here, we used LO (longitudinal optical) phonons and alloy scattering to the elucidate charge-carrier mobilities in the FA0.83Cs0.17Pb(I1-xBrx)3 hybrid perovskite system. It was found that these scattering mechanisms provided very good quantitative agreement with the experimental results, between 11-40 cm2 V-1 s-1. Our findings provide new insights into charge transport scattering in lead mixed-halide hybrid perovskites and pave the way toward design of novel semiconductor alloys for solar cell applications.