Ataxia Telangiectasia Mutated (ATM) kinase deficiency results in cancer susceptibility and drug sensitivity. Deficiency in either the BRCA1 interacting A complex or XRCC4/Ligase 4 confers resistance to Topoisomerase I or PARP1 inhibitors in ATM-deficient cells. This suggests that BRCA1-A directs toxicity to fork-damaging agents in ATM mutated cells vis-a-vis illegitimate end-joining. Here, we show that ATM inhibition triggers combined SUMO and ubiquitin mediated BRCA1-A damaged fork recognition to restrict end-resection and cause Topoisomerase I inhibitor hypersensitivity. BRCA1-A deficient cells display elevated chromatin accessibility and nuclease activity at damaged forks, coupled with restored resection and drug resistance. Electron microscopy evidence demonstrates that ATM inhibition prevents replication fork reversal, which is restored by BRCA1-A loss to generate substrates for end resection. These findings reveal that BRCA1-A enforces a restrictive chromatin state to suppress the genesis of resection substrates, implicating fork reversal as a key determinant of chemotherapy response in ATM deficient cells.