Chromosomal translocations generate oncogenic fusion proteins in approximately one-third of sarcomas, but how these proteins promote tumorigenesis and the effect of cancer therapies on their function are not well understood. Here, we reveal a molecular mechanism by which the fusion oncoprotein FUS-CHOP promotes tumor maintenance that also explains the remarkable radiation sensitivity of myxoid liposarcomas. We identified novel interactions between FUS-CHOP and chromatin remodeling complexes that regulate sarcoma cell proliferation. One of these chromatin remodelers, SNF2H, co-localizes with FUS-CHOP genome-wide at active enhancers. Following ionizing radiation, DNA damage response kinases phosphorylate the prion-like domain of FUS-CHOP to impede these protein-protein interactions, which are required for transformation. Therefore, the DNA damage response after irradiation disrupts oncogenic targeting of chromatin remodelers required for FUS-CHOP-driven sarcomagenesis. Significance Prion-like domains translocated in cancer have been shown to drive global epigenetic changes that are oncogenic. However, some translocation-driven cancers exhibit dramatic clinical responses to therapy, though the mechanism for these responses are not well-understood. Here we show that ionizing radiation can disrupt oncogenic interactions between a fusion oncoprotein and a chromatin remodeling complex, ISWI. This mechanism of disruption links phosphorylation of the prion-like domain in an oncogenic fusion protein to DNA damage after ionizing radiation and reveals that a dependence on oncogenic chromatin remodeling underlies sensitivity to radiation therapy in myxoid liposarcoma.