Mindfulness has recently shown promise in mental illness treatment and preventative contexts with school-aged young people. However, there is a shortage of studies investigating the effects of school-based mindfulness interventions on young people of a pre-adolescent and early-adolescent age. Therefore, the aim of the present study was to investigate the effects of a regional multi-site school-based mindfulness programme on wellbeing and resiliency in UK school children aged 9–12 years old. A total of 1,138 children who received mindfulness training completed the Resiliency Scale for Children and Adolescents and the Stirling Children’s Wellbeing Scale pre- and post-intervention. Results showed significant improvements following intervention delivery in positive emotional state, positive outlook, and resiliency, with resiliency effects maintained at a six-month follow-up assessment. Findings indicate that mindfulness delivered by school teachers can improve wellbeing and resiliency in children and young people.