The present mixed-methods study aimed at investigating the effects of three writing conditions, including collaborative writing (CW), collaborative prewriting (CPW), and individual writing (IW), on the performance and motivation of 66 Iranian EFL learners in a computer-mediated communication (CMC) context. Data were obtained through a background questionnaire, writing motivation scale, participants' writing performance on pretest and posttest, audio-recorded reflections, and semi-structured individual interviews. The quantitative data were analyzed using one-way ANOVA and paired-samples t-test, while the qualitative data were examined through a three-phase thematic analysis. Based on the obtained results, on the posttest, the CPW group significantly outperformed the IW group. No similar trend, however, was observed between CW and IW groups. In addition, as a result of collaborative writing and prewriting in a CMC context, the participants' writing motivation significantly improved. Further findings as well as relevant pedagogical implications are discussed and future research avenues are outlined.