Importance: Low school preparedness is linked to high school dropout, poor employment, and negative outcomes. Childcare attendance may increase school readiness and foster academic achievement. Objective: To explore whether childcare attendance was associated with academic achievement at the end of compulsory schooling (age 16 years in the UK), whether maternal education level was a moderator, and the benefit-cost ratio of childcare regarding productivity returns of academic achievement. Design, Setting, and Participants: In this cohort study, data were included from the Avon Longitudinal Study of Parents and Children (ALSPAC) born from April 1991 to December 1992 and the UK National Pupil Database for examination results. Data on academic achievement at age 16 years were available for 11843 participants. Data were collected from June 2006 to June 2008, and data were analyzed from September 2019 to May 2020. Exposures: On average, 3.7%, 5.9%, and 90.4% attended childcare full time, part time, and less than 10 hours per week, respectively. Maternal education was assessed by questionnaire during pregnancy. Analyses included weights for population representativeness and propensity score weights to account for parental selection into childcare. Main Outcomes and Measures: Academic achievement was defined as no certificate, Level 1 General Certificate of Secondary Education (GCSE; limited training), or Level 2 GCSE (qualification for academic post-16 education; high school diploma equivalent). Lifetime productivity return estimates were withdrawn from previous economic analysis based on pupil's qualifications. Results: Of 14541 children in the ALSPAC study, 8936 children had complete data on childcare attendance, academic achievement, and maternal education levels. Of these, 4499 (50.3%) were male. Attending childcare was associated with higher probabilities of obtaining a Level 1 or 2 GCSE qualification (Level 1: relative risk, 1.41; 95% CI, 1.16-1.73; Level 2: relative risk, 1.62; 95% CI, 1.30-2.01); however, this association was moderated by the child's maternal education level. When children of mothers with low education attended childcare, their probability of no GCSE qualification went from 28.9% (95% CI, 26.8-31.0) to 20.3% (95% CI, 18.0-22.8), whereas children of mothers with higher education had a probability of no qualification of less than 10% regardless of childcare attendance. The benefit-cost ratio for each £1 (US $1.40) invested in full-time childcare attendance for children of mothers with low education was £1.71 (95% CI, 1.03-2.45; US $2.39; 95% CI, 1.44-3.43) for those who reached a Level 2 GCSE qualification. Conclusions and Relevance: Promoting universal childcare with facilitated access for children of lower socioeconomic backgrounds deserves to be considered as a way to reduce the intergenerational transmission of low academic achievement.