A large body of literature has demonstrated that there are developmental differences in mental health problems. However, less is known about the development of mental health problems in ethnic minority children, particularly at the population level. Using a detailed ethnic classification and nationally representative data from the UK Millennium Cohort Study (n = 18, 521, 49% female, 18% ethnic minority), this study examines ethnic differences in children’s mental health problems and trajectories of mental health from ages 3 to 14 years. Growth curve modeling revealed that ethnic minority children followed different developmental trajectories of internalizing and externalizing problems than white children, either in terms of the mean-level and/or rate of change across age. These differences were not explained by child sex, socioeconomic status, maternal depressive symptoms, and maternal immigrant status, highlighting the need for further research exploring the factors that underpin ethnic inequalities in child mental health.