This mixed-research synthesis examined the short and long-term effects of out-of-school learning (OSL) activities on student achievement and factors affecting these practices. Initially, two metaanalytic reviews examined the short and long-term effects of OSL practices on student achievement under a random-effects model. Then, a thematic synthesis of primary qualitative studies explored the pedagogical factors affecting OSL practices. Finally, results from both phases were integrated through a cross-study synthesis. The study found that OSL significantly improves student achievement, g = 1.062, 95% CI [0.930, 1.194], and achievement retention, g = 1.033, 95% CI [0.759, 1.307]. However, there were considerable within and between-study heterogeneities. The moderator variables of country, discipline area, intervention length, and grade levels significantly contributed to effect size variability in one or both meta-analyses. The thematic synthesis revealed many pedagogical factors and implications that needed consideration when planning for, implementing, and finalizing OSL. Interventions matching the implications drawn from the primary qualitative studies had statistically significant effects on student outcomes in many cases. The study concluded by discussing major findings, conclusions, and implications for future research, teachers, and policymakers.