This study investigated the role of working memory (WM) in the second language (L2) writing performance of young English language learners. It also examined how L2 writing achievement relates to task type and grade level and whether the effect of cognitive abilities varies across different task types and grade level. The participants were 94 young learners (Grades 6 and 7) in Hungary, who performed four writing task types as part of the TOEFL® Junior™ Comprehensive test-battery and completed cognitive tests that assessed their WM functions. Participants scored high on the email writing and integrated Listen-Write tasks. Irrespective of WM functions, on average learners in Grade 7 outperformed those in Grade 6 on the Listen-Write task and the Email task. Students gained lower scores on the non-academic version of an editing task than on most other types of tasks. WM functions had no significant relationship with L2 writing scores, except for the academic editing task. In Grade 7, the effect of WM was not significant on the integrated Listen-Write task, but it resulted in the change of expected score. Learners with high working memory in Grade 6 showed somewhat more consistent performance across tasks than did learners with low working memory.