<p>Children and youth face rising mental health challenges in an increasingly digital world. Universal Tier 1 digital mental health interventions (DMHIs)—low-intensity, high-reach supports for non-clinical populations—have emerged as scalable tools for early-stage needs, but effectiveness remains unclear. This systematic review and meta-analysis of 57 studies (<i>N</i> = 43,973) assessed impacts across emotional, behavioural, social, and cognitive outcomes, and examined whether delivery format (hybrid vs. virtual) and timing (post-intervention vs. follow-up) influenced results. Across outcomes, 9/16 pooled effect sizes were statistically significant, indicating small effects. Hybrid interventions, those incorporating any in-person component, showed more consistent and sustained benefits, with moderate-certainty evidence at follow-up, though most pooled findings were low certainty. Parents of younger children and marginalized children and youth were notably understudied. Future research must prioritize marginalized populations, broaden outcome evaluation, and deliver large-scale, high-quality trials to inform inclusive, sustainable implementation of digital mental health supports.</p><p>INPLASY registration number: INPLASY202490026.</p>