Self-regulatory abilities are robust predictors of important outcomes across the life span, yet they are rarely taught explicitly in school. Using a randomized controlled design, the present study investigated the effects of a 12-week mindfulness-based Kindness Curriculum (KC) delivered in a public school setting on executive function, self-regulation, and prosocial behavior in a sample of 68 preschool children. The KC intervention group showed greater improvements in social competence and earned higher report card grades in domains of learning, health, and social-emotional development, whereas the control group exhibited more selfish behavior over time. Interpretation of effect sizes overall indicate small to medium effects favoring the KC group on measures of cognitive flexibility and delay of gratification. Baseline functioning was found to moderate treatment effects with KC children initially lower in social competence and executive functioning demonstrating larger gains in social competence relative to the control group. These findings, observed over a relatively short intervention period, support the promise of this program for promoting self-regulation and prosocial behavior in young children. They also support the need for future investigation of program implementation across diverse settings.