Inorganic arsenic (iAs) is a ubiquitous drinking water and food contaminant. Chronic exposure to iAs has been linked to diabetes mellitus in many populations. Experimental studies support these associations and suggest mechanisms, but the precise pathogenesis of iAs-associated diabetes is unclear. Efficient metabolism of iAs through methylation, catalyzed by arsenic (+3) methyltransferase (AS3MT), influences the risk of iAs-associated disease. Additionally, higher intake of methyl donor nutrients such as folate and methylcobalamin (B12) has been linked to increased efficiency of iAs metabolism and lowered disease risk. However, while metabolism of iAs facilitates its excretion from the body, iAs metabolites are more toxic than iAs itself. The role of iAs metabolism in iAs toxicity is understudied, especially in cardiometabolic disease. Understanding how iAs metabolism affects iAs-associated diabetes will help design appropriate intervention strategies and improve risk assessment. This project investigated how iAs metabolism influences iAs-associated diabetes. First, we compared metabolite profiles of C57BL6 (WT) with As3mt-KO (KO) mice, which cannot methylate iAs, with and without exposure to iAs. We found that while iAs exposure was linked to very few metabolites, knock-out of As3mt was associated with metabolites in a sex-specific manner. Second, we investigated the effect of iAs exposure at environmentally-relevant doses on glucose metabolism in mice and the role of methyl donor nutrient supplementation. Prenatal exposure to iAs elicited hyperglycemia and insulin resistance, particularly in males. Chronic iAs exposure in adult mice increased insulin resistance only with a low-folate, high-fat diet. Males were more insulin resistant than females; KO mice more than WT mice. In both studies, supplementation with B12 and/or folate protected against iAs-associated impairments in glucose metabolism, despite having a minimal effect on iAs metabolism. Third, we investigated how iAs and its methylated metabolites inhibit insulin secretion in pancreatic β-cells. These arsenicals may be impairing calcium influx by affecting ion channels. Interestingly, methylated metabolites appear to target different mechanisms than iAs. Taken together, this research adds novel knowledge about the effects of environmentally-relevant iAs exposures on glucose metabolism in mice, the underlying mechanisms, and the role of iAs metabolism and methyl donor nutrients in metabolic health.