Quality of life is typically reduced in children with mental health problems. Understanding the relationship between quality of life and mental health problems and the factors that moderate this association is a pressing priority. This was a cross-sectional study involving 45,398 children aged 8-13 years from 880 schools in England. Self-reported quality of life was assessed using nine items from the KIDSCREEN-10 and mental health was assessed using the Me and My School Questionnaire. Demographic information (gender, age, ethnicity, socio-economic status) was also recorded. Quality of life was highest in children with no problems and lowest in children with both internalising and externalising problems. There was indication that quality of life may be reduced in children with internalising problems compared with externalising problems. Approximately 12 % children with mental health problems reported high quality of life. The link between mental health and quality of life was moderated by gender and age but not by socio-economic status or ethnicity. This study supports previous work showing mental health and quality of life are related but not synonymous. The findings have implications for measuring quality of life in child mental health settings and the need for approaches to support children with mental health problems that are at particular risk of poor quality of life.