Considering the undeniable importance of examining the role of domain- and skill-specific individual difference factors in second-language (L2) writing research, this study examined the possible roles of English-as-a-foreign-language (EFL) learners’ ideal L2 writing self and growth L2 writing mindset in their L2 writing grit, which may in turn contribute to their L2 writing achievement (WA). Data were collected from 532 English-major students selected via convenience sampling by administering a questionnaire to measure their growth L2 writing mindset, ideal L2 writing self, and L2 writing grit. The International English-Language Testing System (IELTS) academic writing tasks 1 and 2 were used as measures of the learners’ L2 WA. The psychometric properties of the scales were investigated and verified through confirmatory factor analyses. Structural equation modeling (SEM) results revealed that ideal L2 writing self and L2 writing grit directly predicted L2 WA. Additionally, growth L2 writing mindset and ideal L2 writing self predicted L2 WA through the mediation of L2 writing grit. This study highlighted the importance of domain- and skill-specific individual difference factors in L2 WA and the vital necessity of considering them in L2 writing instruction.