Chromosomal rearrangements of the mixed lineage leukemia (MLL/KMT2A) gene leading to oncogenic MLL-fusion proteins occur in ~10% of acute leukemias and are associated with poor clinical outcomes, emphasizing the need for new treatment modalities. Inhibition of the DOT1-like histone H3K79 methyltransferase (DOT1L) is a specific therapeutic approach for such leukemias that is currently being tested in clinical trials. However, in most MLL-rearranged leukemia models responses to DOT1L inhibitors are limited. Here, we performed deep-coverage short hairpin RNA sensitizer screens in DOT1L inhibitor-treated MLL-rearranged leukemia cell lines and discovered that targeting additional nodes of MLL complexes concomitantly with DOT1L inhibition bears great potential for superior therapeutic results. Most notably, combination of a DOT1L inhibitor with an inhibitor of the MLL-Menin interaction markedly enhanced induction of differentiation and cell killing in various MLL disease models including primary leukemia cells, while sparing normal hematopoiesis and leukemias without MLL rearrangements. Gene expression analysis on human and murine leukemic cells revealed that target genes of MLL-fusion proteins and MYC were suppressed more profoundly upon combination treatment. Our findings provide a strong rationale for a novel targeted combination therapy that is expected to improve therapeutic outcomes in patients with MLL-rearranged leukemia.