Portfolio assessment (PA) as an assessment "for" learning (AfL) alternative has been under-represented in second/foreign language acquisition (SLA) research literature. This study examined the potential impacts of electronic PA (e-PA) on English-as-a-Foreign-Language (EFL) learners' engagement modes in descriptive and narrative genres of writing on Moodle™. To do so, 56 university students were non-randomly selected and assigned into two intermediate-level EFL cohorts. In a pretest-mediation-posttest study, descriptive and narrative writing tasks completed by two groups were subjected to teacher feedback, student reflection logs, and subsequent revision every week. Results of repeated measures ANOVA indicated significant progress in lower-level skills (sentence structure, word choice/grammar, mechanics), and moderate progress in higher-level skills (organization, development) in both groups' genre-based writing. Results of one-way ANCOVA reported the notable pretest-to-posttest achievement by both groups with no intergroup statistical differences. The content of students' reflection logs was inductively analyzed for their behavioral, emotional, and cognitive modes of engagement in e-PA. Qualitative data analysis indicated similar writing time intervals and recurrence of revisions as the behavioral mode of both groups. Participants also expressed novelty, low anxiety, and enjoyment as their emotional experiences. In terms of their cognitive experience, the majority agreed upon the applicability of teacher feedback and positive perception of writing improvement in e-PA. Yet, they were critical to regular mismatches between the scopes of teacher assessment and self-assessment, as well as teacher linguistic bias towards certain writing features. Several pedagogical implications of the study promote the facilitating role of e-PA in genre-based academic writing and e-learning contexts.