Anxiety disorders are common mental health problems amongst youth with harmful impacts often extending into adulthood. Mindfulness-based interventions (MBIs) have become increasingly popular for addressing mental health issues, particularly in schools; however, it remains unclear how effective they are for reducing youth anxiety. This meta-analysis aimed to evaluate the efficacy and effect moderators of MBIs on anxiety outcomes in children and adolescents. Eligible studies were published randomised controlled trials (RCTs) of MBIs conducted with participants aged 18 years or younger, investigating anxiety outcomes using a well-validated anxiety scale. A systematic search of RCTs published through to February 2019 identified 20 studies for inclusion (n = 1582). A random effects model was used to synthesise MBI effects. Stratified meta-analyses as well as individual, random effects meta-regressions were performed to examine how effects varied by age group, intervention setting, control type, research location, and intervention dosage. Although, across all studies, there was a small beneficial effect of MBIs on anxiety post treatment (d = 0.26), this was significantly moderated by research location, with RCTs conducted in Iran producing large effects (d = 1.25), and RCTs conducted in Western countries demonstrating no significant beneficial effect compared to controls (very small, d = 0.05). Effects were non-significant at follow-up assessment points. Post-treatment effects were significant for MBIs conducted with children (d = 0.41) and for MBIs when compared to passive controls (d = 0.33), but non-significant for adolescents (d = 0.21), for MBIs conducted in schools (d = 0.30) and in clinics (d = 0.13), and when MBIs were compared to active controls (d = 0.12). Results suggest that MBIs are likely to have a small to medium, yet temporary effect in reducing anxiety symptoms in children (not adolescents), but amongst Western youth populations the most likely outcome, from RCTs to date, is that MBIs produce no beneficial effect in anxiety reduction. Results revealed a lack of evidence to support investment in school-based MBIs to address youth anxiety.