Involving parents and children in mindfulness-based interventions may holistically benefit family well-being. This meta-analysis systematically reviews and synthesizes the effects of mindfulness-based parallel-group (MBPG) interventions, which simultaneously involve parent and child, on family functioning, and the mental health of parents and children. Methods: A total of 20 relevant studies were identified from 14 databases. The overall intervention effect size was estimated by pooled standardized mean difference. Moderator analyses were performed to explain the variability in intervention effects. Risk of bias and publication bias were also assessed. Results: MBPG interventions showed minor-to-small positive effects on family functioning (d = 0.182, 95% CI [0.045, 0.319]), parental mental health (d = 0.238, 95% CI [0.110, 0.365]), and child mental health (d = 0.325, 95% CI [0.137, 0.513]). The effects of MBPG interventions on child mental health varied significantly by child age, child gender, recruitment setting, type of parent group, other activities in child group, other activities in parent group, and study design. Conclusions: MBPG interventions show promising effects in improving mental health of both parents and children as well as in improving overall family functioning. However, significant variations exist in characteristics of participants, interventions, and study designs. Given the limited evidence currently available, more studies are needed to assess the determinants of effectiveness in MBPG interventions. Protocol Registration: PROSPERO #CRD42020164927 © 2021, The Author(s).