Although there is robust evidence of the benefits of attachment-based parenting interventions, limited research has examined their impact on dyadic mutuality and toddler behavior problems. Given the central question in prevention research of what works for whom, and the documented relation of maternal psychological risk to parenting and intervention response, it is important to consider the moderating role of maternal psychological risk in the efficacy of attachment-based interventions. The current study extends prior research on a randomized controlled trial of Attachment and Biobehavioral Catch-up (ABC) by examining its impact on dyadic mutuality and the moderating role of maternal psychological risk in ABC's impact on dyadic mutuality and toddler behavior problems. ABC (10 sessions) was provided as a supplement to Early Head Start (EHS) for a sample of predominantly low-income Latinx families. Control families received home-based EHS plus 1 book per week for 10 weeks. We administered a psychosocial interview and video-recorded parent-toddler interaction pre- and post-intervention. Using intent-to-treat analyses, we found main effects of ABC on dyadic mutuality. We conducted latent class analysis to identify patterns of interrelationships among indicators of baseline maternal risk exposure to characterize a latent risk factor and used this factor to examine the moderating role of maternal psychological risk in ABC's impact on dyadic mutuality and toddler behavior problems. ABC seemed to be particularly beneficial for the development of positive, synchronous dyadic interactions and for reduction of toddler behavior problems in higher-risk EHS families. Findings are discussed in the context of designing and evaluating preventive interventions, with a specific focus on families at psychological risk.