The aim of the present study was to assess the effectiveness of a mindfulness-based social–emotional learning program (MindKinder) on the psychosocial adjustment and neuropsychological development of kindergarten children. The experimental cohort comprised 74 kindergarten children, a cohort obtained through an incidental non-probability sampling method or by accessibility. The children were randomly assigned to control (n = 26; 35.14%) or experimental groups (n = 48; 65.86%). The age of children ranged from 4 to 6 years (M = 5.08, SD = .37). The study followed a quasi-experimental design with repeated pre-test, post-test, and follow-up (after 6 months) measurements with a control group. The MindKinder program was implemented during school time. Results confirmed that the program produced a significant improvement in the psychosocial adjustment and neuropsychological variables in the experimental group. Results showed a significant reduction in the scores of some global maladaptive dimensions among the experimental group: behavioral symptoms index and externalized and academic problems. Likewise, the results show a significant increase in the scores for the experimental group in the dimensions of global development, non-verbal development, visual perception, and attention. Our findings suggest that mindfulness programs may be effective in promoting psychosocial adjustment and improving neuropsychological variables in young children. Implications are discussed, and recommendations are made for future research.