Objective: To examine the association between childhood ADHD and lifestyle risk factors (physical inactivity and high fast-food consumption) from adolescence to early adulthood. Methods: This retrospective cohort study used secondary data from Wave I to III of the National Longitudinal Study of Adolescent Health (Add Health) in a national representative sample of adolescents (n = 6,814). Multivariable Poisson regression and Generalized Estimating Equation were used to estimate adjusted relative risks (aRRs) and corresponding 95% confidence intervals (95% CIs), controlling for confounders. Results: Childhood ADHD was associated with an increased risk of high fast-food consumption (aRR = 1.49, 95% CI [1.23, 1.80]) in early adulthood, but not in adolescence (aRR = 1.11, 95% CI [0.90, 1.37]), after adjusting for confounders. The associations between childhood ADHD and physical inactivity were not statistically significant from adolescence to early adulthood. Conclusion: Findings of this study extend the current understanding of the impact of childhood ADHD on greater risk of fast-food consumption, which underscores the importance to develop interventions for promoting healthy diet in children with ADHD.