Scintillators attract wide research interest for their distinct applications in radiation detection. Elpasolite halides are among the most promising scintillators due to their high structural symmetry and good scintillation performance. A better understanding of their underlying scintillation mechanism opens up possibilities in scintillator development. In this work, we employ a variety of experimental techniques to study the two mixed-anion elpasolites Cs2NaRBr3I3 (R 1⁄4 La, Y). The emission of intrinsic Cs2NaRBr3I3 with a light yield ranging from 20 000 to 40 000 ph=MeV is dominant by self-trapped exciton emission. Partial substitution of R with Ce introduces a competing emission, the Ce3þ 5d-to-4f radiative transition. Ab initio calculations are performed to investigate the electronic structures as well as the binding energies of polarons in Cs2NaRBr6. The calculated large self-trapped exciton binding energies are consistent with the observed high light yield due to self-trapped exciton (STE) emission. The unique electronic structure of halide elpasolites as calculated enhances the STE stability and the STE emission. The highly tunable scintillation properties of mixed-anion elpasolites underscore the role of their complex scintillation mechanism. Our study provides guidance for the design of elpasolite scintillators with exceptional energy resolution and light yield desirable for applications.