Conduct problems can develop into behavior disorders and put children at risk for other mental health problems. Parenting interventions have been shown to successfully reduce conduct problems and are often expected to prevent the development of broader mental health problems. Few studies have evaluated the longer-term and broader effects of these interventions. To what extent are parenting intervention effects sustained in the years after the intervention? And do effects pertain to conduct problems specifically, or do they also affect broader aspects of children’s mental health? We used a randomized controlled trial to assess the longer-term (2.5 years) effects of the Incredible Years parenting intervention on children’s conduct problems in an indicated prevention setting (N = 387; 79% retention rate). Using a multi-method (survey and computerized tasks) and multi-informant (parents, teachers, and children) approach, we tested whether initial effects on conduct problems were sustained, and whether Incredible Years had broader effects on children’s peer problems, emotional problems, attention-deficit/hyperactivity disorder (ADHD) symptoms, attention and inhibition deficits, and service use. Incredible Years, relative to control (no intervention), led to sustained reductions in parent-reported conduct problems (Cohen’s d = 0.31), but not teacher- and child-reported conduct problems. There were no broader benefits: Incredible Years did not reduce children’s peer problems, emotional problems, ADHD-symptoms, attention and inhibition deficits, or their service use. Improvements in parents’ perceptions of child conduct problems sustained until 2.5 years later. Our findings do not show benefits of Incredible Years as a preventive intervention for children’s broader mental health.