Poly (ADP-ribose) polymerase inhibitors (PARPis) are clinically effective predominantly for BRCA-mutant tumors. We introduce a mechanism-based strategy to enhance PARPi efficacy based on DNA damage-related binding between DNA methyltransferases (DNMTs) and PARP1. In acute myeloid leukemia (AML) and breast cancer cells, DNMT inhibitors (DNMTis) alone covalently bind DNMTs into DNA and increase PARP1 tightly bound into chromatin. Low doses of DNMTis plus PARPis, versus each drug alone, increase PARPi efficacy, increasing amplitude and retention of PARP1 directly at laser-induced DNA damage sites. This correlates with increased DNA damage, synergistic tumor cytotoxicity, blunting of self-renewal, and strong anti-tumor responses, in vivo in unfavorable AML subtypes and BRCA wild-type breast cancer cells. Our combinatorial approach introduces a strategy to enhance efficacy of PARPis in treating cancer.