The quality of parenting is recognised as an important determinant of children’s mental health. Parenting interventions typically target high-risk families rather than adopting a universal approach. This study examined the population impact of the Triple P Positive Parenting Programme on the prevalence of children’s social, emotional, and behavioural problems. A propensity score matching difference-in-differences method was used to compare intervention and comparison regions matched on socio-demographic characteristics in midlands Ireland. The pre-intervention sample included 1501 and 1495 parents of children aged 4–8 years in the intervention and comparison regions respectively. The post-intervention sample included 1521 and 1544 parents respectively. The primary outcome measure was parental reports on the Strengths and Difficulties Questionnaire. There were some significant reductions in the prevalence rates of social, emotional, and behavioural problems in the intervention regions compared to the comparison regions. Children in the intervention sample experienced lower total difficulties, emotional symptoms, and conduct problems than children in the comparison sample, and they were less at risk of scoring within the borderline/abnormal range for total difficulties, conduct problems, and hyperactivity. The programme reduced the proportion of children scoring within the borderline/abnormal range by 4.7% for total difficulties, 4.4% for conduct problems, and 4.5% for hyperactivity in the total population. This study demonstrated that a universal parenting programme implemented at multiple levels using a partnership approach may be an effective population health approach to targeting child mental health.