We Learn Together is a 20-week, low-cost infant/toddler school-readiness intervention developed to provide instructional content and supportive tools for teachers to be more explicit and intentional in interactions with children to support early development. Short-term effects were established in a previously published real-world effectiveness randomized controlled trial. In the present quasi-experimental study, we examined the extent to which the effects remained approximately 1 year after the intervention. Because the original control group eventually also got the intervention, we compared mandatory municipal language assessment outcomes of 2,327 children in the treatment group to matched nontreatment children. The intervention effect was maintained at similar magnitudes at the 1-year follow-up for oral language skills (effect sizes = 0.09-0.14), and spillover effects were found for rhyming skills (effect size = 0.12).