Background Positive mental health (not simply absence of mental disorder) may hold potential for fostering resilience and reducing risks of poor health and well-being. However, it is challenging to define and measure. One promising conceptualisation is mental health competence (MHC), comprising age-relevant developmental tasks and abilities. To date MHC has not been investigated in UK children.

Methods We developed a measure of MHC in the UK Millennium Cohort Study (MCS) (~18 000 children born 2000–2002) using seven items addressing learning skills and prosocial behaviours, reported by mothers at age 11 (n=12082). Latent class analysis (LCA) was used to identify classes of MHC, and children were assigned to the class they had the highest probability of belonging to. We investigated associations between MHC classes and child’s physical health (healthy weight, overweight or obese, based on measured heights and weights, using International Obesity Task Force cut-offs; maternal report of unintentional injuries since age 7 [none, 1, 2+]) and poor mental well-being (low self-esteem [SE] and low life-satisfaction [LS] reported by the child; maternal report of child’s emotional problems [EP], all dichotomised at the bottom decile of scores). Relative risk ratios (aRRR, 95% CI) and odds ratios (aORs, 95% CI) were used to examine three-category and binary outcomes respectively, adjusting for potential confounding. Survey weights accounted for sample design and attrition. Analyses were undertaken in Stata/SE 13.1.

Results Four classes were identified (ranging from high to low MHC): “High learning skills and high prosocial behaviour” (37%), “Moderate learning skills and high prosocial behaviour” (36%), “Moderate learning skills and moderate prosocial behaviour” (19%), and “Low learning skills and moderate prosocial behaviour” (8%).

Risks of 2+injuries were raised in “Moderate learning and high prosocial behaviour” (aRRR: 1.4 [95%CI:1.2–1.7]) and “Low learning skills and moderate prosocial behaviour” (aRRR: 1.4 [95%CI:1.1–1.9]) as compared to “High learning and high prosocial behaviour”. Associations with MHC were absent or weak for single injury, overweight and obesity.

Compared to “High learning skills and high prosocial behaviour”, odds of poor mental well-being were elevated for children from all other classes, with highest odds for “Low learning skills and moderate prosocial behaviour” (SE: aOR: 2.9 [95%CI:2.3–3.6]; EP: aOR: 4.2 [95%CI:3.4–5.1]; LS: aOR: 3.0 [95%CI:2.4–3.7]).

Conclusion Lower MHC, using a composite measure developed in a representative sample of UK children, was associated with injuries, low self-esteem and life-satisfaction, and emotional problems, but not overweight and obesity. Identifying and promoting MHC at the population-level may provide an opportunity to improve health in children and young people.