To examine the efficacy of the Good Behavior Game (GBG) in improving children’s reading attainment, and the extent to which this varies as a function of cumulative intervention intensity (dosage) and timing of outcome measurement. A 2-year cluster-randomized controlled trial was conducted. Seventy-seven primary schools from three regions in England were randomly assigned to intervention and control groups. Children (N = 3084) aged 67 at baseline were the target cohort. The GBG is an interdependent group-contingency behavior management strategy used by teachers in elementary schools. Reading attainment was assessed via national teacher assessment scores at baseline, and the Hodder Group Reading Test at post-test and 1-year post-intervention follow-up. Dosage was assessed using a bespoke online GBG scoreboard system. Multi-level intent-to-treat (ITT) and complier average causal effect (CACE) estimation were utilized. At post-test, no effects of the GBG on children’s reading attainment were found in either the ITT or CACE models. At 1-year follow-up, results remained null in the ITT model, but a significant intervention effect was found among moderate compliers (Δ = 0.10) in the CACE model. The GBG can produce measurable improvements in children’s academic attainment, but these effects may take time to become apparent and are contingent upon implementation dosage falling within an optimal range. The project was supported by funding from the Education Endowment Foundation and the National Institute for Health Research. ISRCTN: 64152096.