Background: Parenting programmes are effective in improving child behaviour and parental well-being, but long follow-up studies of universally offered programmes are scarce. Methods: A cluster randomized controlled trial was conducted to assess the costs and effects of Triple P levels 2–3 on child externalizing behaviours and parental mental health. The programme was offered universally to parents of preschoolers (self-selection allowed). Preschools were randomized to Triple P or a waitlist control. Health outcomes were reduction in externalizing behaviours measured on the Eyberg Child Behaviour Inventory-22 and improvement in parental mental health measured on the Depression Anxiety Stress Scales collected at baseline, 6-, 12- and 18-month follow-up. Child outcomes were based on 355 children aged 3–5 years (child sample) and parental outcomes on 759 parents (parental sample) with baseline data. Costs were collected from a municipality perspective, including 312 children and 488 parents with baseline data in the intervention preschools. Results: Sixty-seven (29%) parents attended the intervention. Triple P showed no significant improvement in child externalizing behaviours or parental mental health at either of the follow-up points. Triple P had an average yearly total cost of 3007 Swedish Krona (SEK) (€323) per child or 1922 SEK (€207) per parent. Running Triple P cost 227 SEK (€24) per child or 145 SEK (€16) per parent yearly. Conclusion: Offering low intensity levels of Triple P with 29% attendance may not be a reasonable use of public resources, as no evidence of improvement in child externalizing behaviours or parental mental health was found.