More research is needed to understand the effects of school-based mindfulness programs in the years before adolescence, which represent a critical juncture and transitional period of development. The present study investigated mindfulness with elementary school students using random assignment and objective measures. The sample included 292 5th graders from 21 classrooms randomly assigned to an 8-week mindfulness training or wait-list control group. Students were assessed at pre- and post-intervention on behavioral measures of executive functioning and teacher-rated social emotional competence, along with end of year social-emotional learning report card grades. Analyses using hierarchal linear modeling found that students in the intervention group demonstrated significant gains on a computerized task of cognitive flexibility, and end of year social-emotional learning grades controlling for prior year grades, but not teacher reported social emotional competence. Taken together these findings point to a simple, yet promising method for bolstering students' cognitive and social emotional skills. [This paper will be published in "Applied Developmental Science."]