While working memory (WM) is a powerful predictor for children's school outcomes, autistic children are more likely to experience delays. This study compared autistic children and their neurotypical peers' WM development over their elementary school years, including relative growth and period of plasticity. Using a nationally-representative dataset, latent growth models were built to examine periods of high plasticity and the relationship between children's performance upon school entry and their relative growth. While both groups made steeper gains during the early school years, autistic children's period of highest plasticity was prolonged by 1 year, which suggests a larger window for interventions. Further, autistic children who started kindergarten with poorer WM were more likely to make rapid growth during the last 3 years of elementary school, which is when their neurotypical peers' development started to plateau. Findings should prompt various stakeholders to examine interventions and instructions to maximize autistic children's growth in WM. Further, the continued support and monitoring by educators throughout autistic children's late childhood can be particularly beneficial for the "late-bloomers."