Adults demonstrate difficulty and pronounced variability when developing second language (L2) grammatical knowledge and reading skills. We examine explanations in terms of individual differences in working memory (WM). Despite numerous studies, the association between WM and adult second language (L2) acquisition remains unclear, and longitudinal studies are scarce and contradictory. This study investigates whether WM affects L2 grammar and reading development in beginning classroom learners, using WM tests with (Waters and Caplan’s 1996 test) and without (Daneman and Carpenter’s 1980 test) a demanding processing task. In Experiment 1, 82 beginning first language (L1) English learners of Spanish completed Daneman and Carpenter’s test, and grammar and reading pretests and posttests one year apart. In Experiment 2, 330 beginning English learners of Spanish completed the same tests as in Experiment 1 and Waters and Caplan’s test. The results reveal that only Waters Caplan’s test (response time, recall span) yielded WM effects, and that response time (processing) negatively correlated with recall span (storage). These findings reveal longitudinal WM effects on L2 grammar and reading development at early acquisition stages, support resource-sharing WM models, and urge scholars to adopt WM tests with a processing task performed under timed conditions, and to analyse response time.