The present study adopted a mixed-methods research design and explored the role of a set of cognitive (i.e., aptitude and working memory) and motivational (i.e., self-regulatory capacity and self-efficacy beliefs) individual difference variables in the writing quality and composing behavior of 78 Iranian undergraduate EFL learners. The necessary data were collected through a series of instruments and both quantitative (e.g., multiple regression and t-tests) and qualitative (e.g., narrative construction and qualitative comparative analysis) techniques were used to analyze the data. The results of these analyses indicated that the construct of foreign language aptitude had the highest level of correlation and contributory potential to account for the writing competence of the learners. The composing process of learners with different individual characteristics was also compared and it was found that learners with high self-regulation capacity orchestrated and managed their composing behavior in more effective ways compared to their less self-regulated counterparts. Moreover, the narratives and qualitative comparative analysis provided some insights about how various individual characteristics might affect the composing behavior of the individual learners. Finally, it was suggested that consideration of individual differences in writing can reveal more subtle information about the causes of strengths and weaknesses of different learners and may enable the teachers to design and implement more effective instructions targeting their learners’ individual needs