Introduction: Determining the population-based scope and stability of eating, activity, and weight-related problems is critical to inform interventions. This study examines: (1) the prevalence of eating, activity, and weight-related problems likely to influence health; and (2) the trajectories for having at least one of these problems during the transition from adolescence to adulthood. Methods: Project EAT I-IV (Eating and Activity in Teens and Young Adults) collected longitudinal survey data from 858 females and 597 males at four waves, approximately every 5 years, from 1998 to 2016, during the transition from adolescence to adulthood. Analyses were conducted in 2017–2018. Measures included high fast-food intake (≥3 times/week), low physical activity (<150 minutes/week), unhealthy weight control, body dissatisfaction, and obesity status. Results: Among females, the prevalence of having at least one eating, activity, or weight-related problems was 78.1% at Wave 1 (adolescence) and 82.3% at Wave 4 (adulthood); in males, the prevalence was 60.1% at Wave 1 and 69.2% at Wave 4. Of all outcomes assessed, unhealthy weight control behaviors had the highest prevalence in both genders. The stability of having at least one problem was high; 60.2% of females and 34.1% of males had at least one problematic outcome at all four waves. Conclusions: The majority of young people have some type of eating, activity, or weight-related problem at all stages from adolescence to adulthood. Findings indicate a need for wide-reaching interventions that address a broad spectrum of eating, activity, and weight-related problems prior to and throughout this developmental period.