Children from low-income families experience worse mental health than their wealthier peers. In this study, we quantified income inequalities in child mental health from early childhood to adolescence and identified key contributing factors explaining observed inequalities. Using data from the UK Millennium Cohort Study, we followed 5,667 children aged 3 to 17. We analysed overall mental health problems and their internalising and externalising domains, assessed using the Strengths and Difficulties Questionnaire. We applied a concentration index approach to quantify these inequalities, incorporating both relative and absolute inequality measures. We then decomposed the concentration index to analyse the contribution of various risk factors at different ages. We found inequalities across all mental health domains at every age, with poor mental health concentrated among children from low-income families, and an increasing inequality in internalising problems over time. The decomposition analysis showed that maternal depression and child-parent relationships were key contributors to these inequalities. These findings highlight the importance of addressing income inequalities in child mental health. Reducing inequalities in maternal depression and child-parent relationships may help reduce the mental health gap between children from lowand high-income families.