The Social-Emotional Prevention Program (SEP) is a multi-focused intervention combining universal and indicated intervention strategies delivered face-to-face and through the use of technology. The study's objectives were to assess comparatively SEP effectiveness as a function of preschoolers' baseline risk of externalizing problems and to evaluate its hypothesized mechanisms of action. A randomized controlled trial (RCT) was conducted with five schools assigned to either the intervention or to a comparison condition (wait-list control). Structural equation models (SEM) and complier average causal effects (CACE) were conducted to evaluate program effectiveness and mechanisms of change. Research Findings: SEP participation was associated with improved social-emotional competence irrespective of baseline risk and at-risk preschoolers were more likely to exhibit reductions in externalizing problems. However, perceptions about which risk group benefited more varied as a function of rater. Patterns for SEP mechanisms of action suggest that these were similar across risk status with both child emotion regulation and parental emotion socialization practices emerging as paths to improved social-emotional competencies. Practice and Policy: The present findings add to the extant evidence suggesting that multi-focused programs combining universal and indicated prevention strategies could be successfully delivered in schools to promote social-emotional learning.