Concern about the use of nanomaterials has increased significantly in recent years due to potentially hazardous impacts on human health. Mast cells are critical for innate and adaptive immune responses, often modulating allergic and pathogenic conditions. Mast cells are well known to act in response to danger signals through a variety of receptors and pathways including IL-33 and the IL-1-like receptor ST2. Here, the involvement of mast cells and the IL-33/ST2 axis in pulmonary and cardiovascular responses to multi-walled carbon nanotube (MWCNT) exposure are examined. Toxicological effects of MWCNTs are observed only in mice with a sufficient population of mast cells and are not observed when mast cells are absent or incapable of responding to IL-33. Our findings establish for the first time that mast cells and the IL-33/ST2 axis orchestrates adverse pulmonary and cardiovascular responses to an engineered nanomaterial, giving insight into a previously unknown mechanism of toxicity. This novel mechanism of toxicity could be used for assessing the safety of engineered nanomaterials and provides a realistic therapeutic target for potential nanoparticle induced toxicities.