Child-caregiver attachment is important for healthy child development and is often targeted by relationship-based parenting interventions for young children. To assess the efficacy of these interventions, attachment must be measured accurately at multiple timepoints across the toddler years, coinciding with different stages of development. Among others, populations referred for treatment of externalizing behavior problems are of particular importance here. The Strange Situation Procedure (SSP) and Attachment Q-Set (AQS) are empirically-validated measures of child-caregiver attachment with unique strengths and weaknesses. However, there are no published investigations on concurrent relations between the two measures using clinically-referred, mental health populations. Previous research has reached mixed conclusions when comparing the observer-report AQS and SSP for children from other populations and based on child age. Using a clinical sample of 69 Australian mother-toddler dyads referred for disruptive behavior problems, this study examined associations among behavior problems, dichotomous SSP classifications of security and organization, AQS security scores, and child age. In line with hypotheses, a small to medium negative correlation between AQS security and externalizing behavior was found. Unexpectedly, no significant associations were found between dichotomous SSP security and externalizing behavior nor between SSP organization and externalizing behavior. Although AQS security scores and SSP classifications were not significantly correlated for the sample as a whole, there was a moderate positive correlation between the two measures for children aged 19–25 months. Implications for future research measuring attachment in this population, with relevance for relationship-based parenting intervention outcome studies are discussed.