The DOT1L enzyme is hijacked by MLL-Fusion oncoproteins (MLL-FP) to aberrantly deposit H3K79methylation at key target genes to drive leukemogenesis. MLL-FP activity can be disrupted by blocking its chromatin occupancy with MLL-Menin inhibitors, or by blocking its biochemical activity through inhibition of DOT1L. Despite these inhibitors having potent anti-leukaemia effects, our understanding of how Menin and DOT1L contribute to MLL-FP function remains incomplete. Here, using functional genomics, we uncover a key role for PRC1.1 in the efficacy of Menin and DOT1L inhibitors by mediating the potent silencing of MLL-FP target genes across a range of in vitro and in vivo MLL-FP-models. Mechanistically, Menin inhibition elicits a PRC1.1-dependent selective induction of H2AK119ub at key MLL-FP target genes, while DOT1L inhibition leads to a more widespread increase in H2AK119ub. Consistent with these differences, we demonstrate that the increase in PRC1.1 activity is specifically associated with loss of H3K79me2 rather than eviction of the MLL-FP or reduction in transcription. In addition, temporal analyses after Menin inhibition revealed the delayed induction of H2AK119ub, which correlated with the passive turnover of H3K79me2, suggesting that DOT1L provides a memory of activation. Importantly, this interplay is conserved across diverse cell contexts suggesting a fundamental role for DOT1L in protecting genes from rapid PRC1-mediated repression. Upon this switch in chromatin state, Polycomb target genes undergo stable PRC2-mediated repression following Menin inhibition, leading to the irreversible commitment of leukaemia cells towards differentiation and proliferation arrest even after early drug withdrawal. Taken together, our findings demonstrate that Polycomb repression is required for the efficacy of these differentiation therapies and highlights why DOT1L is hijacked in MLL-leukaemia with important implications for future therapeutic strategies employing Menin inhibitors.