The cytokine IL-15 is required for Natural Killer (NK) cell homeostasis, however the intrinsic mechanism governing this requirement remains unexplored. Here, we identify the absolute requirement for myeloid cell leukemia sequence-1 (Mcl1) in the sustained survival of NK cells in vivo. Mcl1 is highly expressed in NK cells and regulated by IL-15 in a dose-dependent fashion via STAT5 phosphorylation and subsequent binding to the 3’UTR of Mcl1. Specific deletion of Mcl1 in NK cells results the absolute loss of NK cells from all tissues owing to a failure to antagonizing pro-apoptotic proteins in the outer mitochondrial membrane. This NK lymphopenia results in mice succumbing to multi-organ melanoma metastases, being permissive to allogeneic transplantation and being resistant to toxic shock following polymicrobial sepsis challenge. These results clearly demonstrate a non-redundant pathway linking IL-15 to Mcl1 in the maintenance of NK cells and innate immune responses in vivo.