In New Zealand, a number of studies have documented that New Zealand Māori are at increased risk for a range of adverse developmental outcomes compared to non-Māori. Social scientists have offered a range of explanations for these ethnic differences, but often lack the necessary data to adequately test these explanations. This study used data from a long-term longitudinal study of a birth-cohort of New Zealand adolescents (N = 983) to examine the associations between ethnicity (New Zealand Māori versus non-Māori), prevalence rates of psychiatric disorder (both internalizing and externalizing) and suicidal ideation at age 15, 16, and 18 years. The study also investigated if any differences in mental health outcomes between the two groups were explained by a range of life-course experiences, including childhood socio-economic disadvantage, childhood family adversity, and adolescent deviant peer affiliation. Parents reported on children’s ethnicity at 14 years of age, and psychiatric disorder was measured in adolescence through both self-reports and parent reports. Generalised estimating equation (GEE) models were fitted to repeated measures data to gauge the strength of the associations between Māori ethnicity and psychiatric disorders during adolescence. These GEE models were then extended in a series of adjustments to control for childhood socio-economic disadvantage, childhood family adversity, and adolescent deviant peer affiliation. Findings indicated that the significant ethnic differences in rates of mental disorder amongst New Zealand adolescents were largely explained by the higher rates of exposure amongst Māori to socio-economic disadvantage during childhood, childhood family adversity, and deviant peer affiliation during adolescence. These findings contribute to our understanding of the effects of adverse life-course experiences on an individual’s risk for poor adolescent mental health outcomes.