Controlled anion mixing in halide perovskites has been shown to be an effective route to precisely tune optoelectronic properties in order to achieve efficient photovoltaic, light emission, and radiation detection devices. However, an atomistic understanding behind the precise mechanism impacting the performances of mixed halide perovskite devices, particularly as a radiation detector, is still missing. Combining high-level computational methods and multiple experiments, here we systematically investigate the effect of chlorine (Cl) incorporation on the optical and electronic properties, structural stability, ion migration, and the γ-ray radiation detection ability of MAPbBr3-xClx. We observe that precise halide mixing suppresses bromide ion migration and consequently reduces the dark current by close to a factor of two, which significantly increases the resistance of the mixed anion devices. Furthermore, reduced carrier effective masses and mostly unchanged exciton binding energies indicate enhanced charge carrier transport for these perovskite alloys. At the atomistic level, modifications to ion migration and charge carrier transport properties improve electronic properties and predominantly contribute to the better response and resolution in high-energy γ-ray detection with MAPbBr3-xClx as compared to MAPbBr3. This study provides a systematic approach to enhance the high-energy radiation detection ability of MAPbBr3-xClx-based devices by understanding the atomistic properties underpinning performance.