Introduction Youth with a history of child maltreatment use substances and develop substance use disorders at rates above national averages. Thus far, no research has examined pathways from maltreatment to age of substance use initiation for maltreated youth. We examined the longitudinal impact of maltreatment in early childhood on age of alcohol and marijuana use initiation, and whether internalizing and externalizing behaviors at age 8 mediates the link between maltreatment and age of substance use initiation. Materials and methods Data were drawn from the Longitudinal Studies of Child Abuse and Neglect (LONGSCAN) at ages 4, 8, 12, and 18. Maltreatment was assessed through reviews of administrative records and youth self-reports. Behavior problems were assessed with the Child Behavior Checklist. Age of substance use initiation was assessed with the Young Adult version of the Diagnostic Interview Schedule for Children. Results Path analyses indicated mediated effects from a history of maltreatment to age at first alcohol and marijuana use through externalizing behaviors. Considering type of maltreatment, direct effects were found from physical abuse to age of alcohol initiation, and mediated effects were found from sexual abuse and neglect to initial age of alcohol and marijuana use through externalizing behaviors. Direct effects for marijuana use initiation and indirect effects through internalizing behavior problems were not significant for either substance. Conclusions Externalizing behavior is one pathway from childhood maltreatment to age of substance use initiation. Services for maltreated youth should incorporate substance use prevention, particularly among those with early externalizing problems. Copyright 2017 Elsevier Ltd