This study investigated the efficacy of INSIGHTS into Children’s Temperament (INSIGHTS) in supporting the behaviors and engagement of low-income kindergarten and first-grade children with high-maintenance temperaments. INSIGHTS is a temperament-based social–emotional learning intervention that includes teacher, parent, and classroom programs. Participants in the study included N =435 children (Mean age = 5.38 SD =0.61) from 22 under-resourced, urban elementary schools who were randomly assigned to INSIGHTS or a supplemental after-school reading program. Sixty-nine children were identified as having a high-maintenance temperament, characterized by low levels of task persistence and high levels of motor activity and negative reactivity. Individual growth modeling showed that children with high-maintenance temperaments in INSIGHTS evidenced faster reductions in disruptive behaviors and off-task behaviors across kindergarten and first grade than their peers in the supplemental reading program. Such children in INSIGHTS also had lower overall levels of both disruptive behaviors and off-task behaviors and higher levels of behavioral engagement than children in the comparison group at the end of first grade. Intervention effects for children with high-maintenance temperaments were partially mediated through improvements in their relationships with their teachers. Implications for social–emotional learning intervention for high-risk children and early educational policy are discussed.