Objectives Poor oral health has been shown to be associated with selected adverse health outcomes. This study assessed the association between untreated dental caries and mortality and examined whether having dental insurance mitigates the risk of mortality among working‐age US adults with dental caries. Methods Analysis used the publicly available linked mortality file for NHANES III, an observational study conducted in 1988‐1994, with follow‐up through December 2015. Propensity score matching was conducted to create similar populations of insured and uninsured adults, resulting in a sample of 4420 matched observations. The Cox proportional hazard model was used to investigate the effect of untreated dental caries and that of dental insurance on risk of all‐cause mortality. The descriptive and final outcome statistical analyses were adjusted for complex sampling technique using weights, strata and cluster variables. Results Adults with untreated dental caries had a higher risk of mortality (HR: 1.33; 95% CI: 1.06‐1.68) than those with no dental caries. Having dental insurance was associated with a lower risk of mortality (HR: 0.73; 95% CI: 0.59‐0.92). An interaction between caries treatment status and dental insurance was not statistically significant. Conclusions Adults with untreated dental caries have a higher risk of mortality, even in the presence of dental insurance. Untreated caries may be an indicator for multiple risk factors, including personal attitudes regarding health and healthcare‐seeking behaviour.