Developmental trauma remains a significant problem as several domains of impairment in children and youth exposed to trauma are affected. The momentum of research is currently focused on the consequences of developmental trauma reflected in disruptions in cognitive, neurological and psychological development which places children and youth at an elevated risk in developing maladaptive coping behaviours. Children and youth who report a history of trauma are typically more likely to engage in non-suicidal self-injury (NSSI) and suicidal self-injury (SSI). This research explores whether social support and cognitive and executive functioning moderate the relationship between developmental trauma history and involvement in NSSI and NSSI+SSI in clinically-referred children and youth. Data was drawn from 10 mental health care agencies across Ontario where there was the capacity to complete an interRAI ChYMH assessment. Data includes 8349 participants between 7-18 years of age with multifaceted health histories. Results indicate that the following predictor variables as a set differentiated between children and youth who engage in NSSI and NSSI+SSI from those who do not engage: sexual or physical abuse, witnessing domestic violence, female gender, and older age. Results revealed that cumulative trauma, male gender, and older age predicted engagement in NSSI and NSSI+SSI. Higher levels of social support and cognitive and executive functioning appeared to function as protective factors for some types of abuse and not others, thus moderating the direct association between developmental trauma and self-injurious behaviours. Findings from this study hold implications for clinicians in assessing child and youth protective factors, developing targeted intervention and prevention strategies for self-harming behaviour, and could improve the consistency and continuity of care of the mental health needs of children and youth