Task-based language teaching is believed to facilitate language learning opportunities that arise when performing tasks. Although the synergies between task and learner variables in this process rose to prominence recently, little has been undertaken to explore the individual difference-task interaction in textual meaning-making activities. This study thus explored how second language (L2) writing performance under different task complexity conditions was impacted by L2 writing willingness to communicate (WTC) and L2 writing proficiency. Participants with upper-intermediate English proficiency were recruited following a within-between-participant factorial research design. The results confirmed that WTC significantly influenced syntactic complexity, accuracy, and fluency in L2 writing, suggesting that conative individual differences (IDs) might play a more prominent role in L2 writing than cognitive IDs. Among the sub-components of WTC, motivational predispositions performed better than emotional, cognitive, and writing-specific features in affecting L2 writing performance. Additionally, WTC played a more pronounced role in the complex task, supporting the claim of Robinson’s cognition hypothesis that ID effects are more evident in complex tasks than simple tasks. However, no interaction between L2 writing WTC and proficiency was found. Theoretical and pedagogical implications were offered on considering both L2 WTC and task complexity in task-based writing instruction.