This paper explored the direct influence of an ongoing, intensive, and structured professional development (PD) within an English-as-second-language (ESL) instructional intervention on (a) teachers’ time allocation in cognitive–academic language proficiency (CALP) and (b) Spanish-speaking English language learners’ (ELLs) CALP development from the second to third grade within a multilevel cross-classified framework. It also examined the mediation effect of teachers’ time allocation. It observed that treatment teachers spent more time in CALP than control teachers as a result of the instructional intervention with PD. In addition, the treatment effect was evident in ELLs’ outcomes, including expressive vocabulary, oral reading fluency, and retell fluency. Finally, the treatment effect was completely mediated through teachers’ time allocation in CALP in the second grade on retell fluency.