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The mutant isocitrate dehydrogenase 1/2 inhibitor (mIDHi) vorasidenib was recently incorporated into clinical treatment guidelines for IDH-mutant gliomas, although its impact on chemoradiation is unclear. Specifically, it is unknown whether upfront mIDHi exposure alters subsequent chemoradiation efficacy. Addressing this critical question has been challenging because of limited clinical data and a paucity of mIDHi-responsive preclinical glioma models. We first established that a genetic mouse model of IDH-mutant astrocytoma developed by our group was responsive to vorasidenib monotherapy. We then used this mouse to address whether mIDHi alters the response to chemoradiation after progression on mIDHi. Mice that received upfront vorasidenib followed by chemoradiation at progression had improved survival compared with control mice receiving vehicle followed by chemoradiation. We then compiled real-world data and early outcomes from 29 patients who were among the first to receive mIDHi followed by radiation with or without chemotherapy. Our study directly addresses uncertainty surrounding therapy sequencing that has emerged after introduction of vorasidenib as a first-line treatment for IDH-mutant glioma. Our empirical preclinical data demonstrate that prior mIDHi treatment enhances chemoradiation sensitivity of IDH-mutant glioma.