Prior studies have shown that birth complications interact with psychosocial risk factors in predisposing to increased externalizing behavior in childhood and criminal behavior in adulthood. However, little is known about the direct relationship between birth complications and externalizing behavior. Furthermore, the mechanism by which the birth complications predispose to externalizing behavior is not well explored. This study aims to assess whether birth complications predispose to early adolescent externalizing behavior and to test whether Intelligence Quotient (IQ) mediates relationships between predictor and outcome variables. We used data from a prospective, longitudinal birth cohort of 1,795 3-year-old boys and girls from Mauritius to test hypotheses. Birth complications were assessed from hospital record data, malnutrition from a pediatric exam at age 3 years, psychosocial adversity from parental interviews at age 3 years, and externalizing behavior problems from parental ratings at age 11 years. We found that babies with birth complications are more likely to develop externalizing behavior problems at age 11. Low IQ was associated with birth complications and was found to mediate the link between early predictors and later externalizing behavior. These prospective, longitudinal findings have potential clinical implications for the identification of early adolescent externalizing behavior and for public health attempts to prevent the occurrence of child externalizing behavior problems.