The study investigated the effect of task complexity (TC) on second language (L2) learners' affective responses and writing performance and how these were modulated by cognitive and affective individual difference (ID) factors. A total of 412 Chinese 8th-grade English-as-a-foreign-language learners completed a working memory (WM) test, a scale measuring trait-like L2 writing enjoyment, a simple (n = 206) or complex (n = 206) argumentative writing task, and posttask scales measuring task-specific enjoyment and motivation. Independent t-test results showed that increased TC contributed to a slight but significant enhancement in task motivation and enjoyment and a significant performance improvement in content and organization but not in language. Structural equation modeling results showed that the cognitive and affective ID factors predicted writing performance jointly: WM and task motivation predicted writing performance consistently in both simple and complex tasks, task-specific enjoyment only played its role in the complex task, and trait-like enjoyment became insignificant in predicting task performance. These findings support the integrated task-mediated cognitive-affective model of L2 writing we propose by synthesizing and extending existing models. This study highlights the need to pay attention to the affective dimension of tasks and suggests TC as an intervention strategy to make a task more motivating and enjoyable.