ObjectivesThis study examined the impact of the Mindfulness-Based Kindness Curriculum (MBKC) on social-emotional, executive function, and academic skills of preschoolers.MethodSixteen preschool and 4K (4-year-old kindergarten) classrooms (245 children, 57.6% ethnically diverse, 69.8% low SES) were randomly assigned to either a curriculum-as-usual (CAU) or MBKC group taught by trained classroom teachers. Measures, as reported by children, teachers, and parents, were collected prior to and after implementation of the MBKC.ResultsTrained classroom teachers effectively implemented the MBKC. Teachers rated MBKC children significantly higher on outcome measures of social-emotional skills (e.g., sharing, prosocial skills, empathy), executive functioning (e.g., planning/organizing, working memory), and academic skills (e.g., physical development, language, math) than CAU children. Parents rated MBKC children as having significantly higher levels of cognitive empathy compared to parents’ ratings of CAU children. MBKC children were significantly more likely to engage in sharing behavior with a sick child, though children’s self-ratings indicated no effect of the MBKC on mindfulness skills or self-efficacy. Unlike previous research, the MBKC did not benefit initially lower functioning children more than initially higher functioning children.ConclusionsConsistent with previous research, children given the MBKC appeared to benefit in terms of higher social-emotional competency, prosocial behavior, and executive functioning. The MBKC proved to be a useful complement to their other social-emotional learning programming. The present study expands the literature on the application of mindfulness with preschool children and highlights important implications of teaching and measuring mindfulness skills in young children, thereby identifying specific issues to address in future studies.PreregistrationThis study was not preregistered.