To assess the impact of New York City's (NYC's) 2014 increase of the minimum legal purchase age (MLPA) for tobacco and e-cigarettes from 18 to 21 years. We performed a difference-in-differences analysis comparing NYC to the rest of New York State by using repeated cross-sections of the New York Youth Tobacco Survey (2008-2016) and to 4 Florida cities by using the Youth Risk Behavior Surveys (2007-2015). Adolescent tobacco use declined slightly in NYC after the policy change. However, this rate of change was even larger in control locations. In NYC, e-cigarette use increased and reported purchases of loose cigarettes remained unchanged, suggesting uneven policy implementation, enforcement, or compliance. Increasing the MLPA to 21 years in NYC did not accelerate reductions in youth tobacco use any more rapidly than declines observed in comparison sites. Public Health Implications. Other cities and states currently raising their MLPA for tobacco may need to pay close attention to policy enforcement and conduct enhanced monitoring of retailer compliance to achieve the full benefits of the policy.