Genes of the mixed lineage leukemia (MLL) family regulate transcription by methylating histone H3K4. Six members of the MLL family exist in humans, including SETD1A, SETD1B and MLL1-MLL4. Each of them plays non-redundant roles in development and disease genesis. MLL1 regulates the cell cycle and the oscillation of circadian gene expression. Its fusion proteins are involved in leukemogenesis. Here, we studied the role of MLL1 in innate immunity and found it selectively regulates the activation of genes downstream of NF-κB mediated by tumor necrosis factor (TNFα) and lipopolysaccharide (LPS). Real-time PCR and genome-wide gene expression profile analysis proved that the deficiency of MLL1 reduced the expression of a group of genes downstream of nuclear factor κB (NF-κB). However, the activation of NF-κB itself was not affected. The MLL1 complex is found both in the nucleus and cytoplasm and is associated with NF-κB. CHIP assays proved that the translocation of MLL1 to chromatin was dependent on NF-κB. Our results suggest that MLL1 is recruited to its target genes by activated NF-κB and regulates their transcription.