The Affordable Care Act required private insurers to cover a set of recommended preventive services without cost-sharing. This included coverage of fluoride varnish (FV) applications without cost-sharing for children aged 1 through 5 during medical visits, an evidence-based treatment that prevents tooth decay. We examined if this coverage mandate was associated with more young children receiving FV. Using the Massachusetts All-Payer Claims Database (2014-2018), we examined the likelihood that a privately insured child received FV during a medical visit in a month. We used a difference-in-differences approach, comparing those included in the coverage mandate (aged 1-5) to those excluded from the mandate (aged 6-9), before and after the mandate was enacted (January 2015). We repeated analyses in children with Medicaid because this mandate may have had spillover effects for this population. Among children aged 1 through 5 years with private insurance, 1-year postmandate the probability of FV receipt in a month increased 0.16 percentage points more relative to December 2014 (premandate) compared with the change among children aged 6 to 9 years (P < .001; 95% confidence interval = 0.1-0.22). When examining spillover to children with Medicaid, the mandate was not associated with a significant increase in the probability of monthly FV receipt 1-year postmandate. This Affordable Care Act mandate requiring coverage of FV without cost-sharing was associated with higher rates of young children receiving FV in medical settings, with the largest result observed among children with private insurance.