MLL4 is an essential subunit of the H3K4 methylation complexes. We report that MLL4 deficiency compromised regulatory T (T reg) cell development and resulted in substantial decreases in H3K4me1 and chromatin interaction at putative enhancers, a remarkable portion of which were not direct targets of MLL4 but were enhancers that interact with MLL4-bound sites. The decrease in H3K4me1 and chromatin interaction at the MLL4-unbound enhancers correlated with MLL4 binding at distant-interacting regions. Deletion of an upstream MLL4 binding site reduced H3K4me1 at the Foxp3 regulatory elements looped to the MLL4 binding site and compromised both thymic T reg and inducible T reg cell differentiation. We show that MLL4 catalyzed H3K4 methylation at distant unbound enhancers via chromatin looping, thus providing a new mechanism of regulating T cell enhancer landscape and impacting T reg cell differentiation.