Adopting a sequential explanatory mixed-methods approach, this study explored the impact of electronic writing portfolio on writing performance in English as a foreign language (EFL) context. Two intact classes were first selected and randomly assigned to an electronic writing portfolio group (22 EFL students) and a conventional (i.e. paper-based) writing portfolio group (23 EFL students). To collect the quantitative data, timed writing tasks were administered before and after the electronic and conventional writing portfolio courses, and to collect the qualitative data, the students' peer writing mediations were tracked during the courses and an individual semi-structured interview was conducted at the end of the electronic writing portfolio course. One-way ANCOVA and Pearson Chi square were applied to analyse the quantitative data and microgenetic method and thematic analysis were run to analyse the qualitative data. The findings showed both electronic and conventional writing portfolio instructions developed the EFL students' writing performance. In addition, the electronic writing portfolio group outperformed its conventional counterpart in developing writing performance. The qualitative data analyses, used to explain the quantitative findings, further revealed the EFL students' microgenetic development of writing content, writing organisation, and language use in the electronic writing portfolio group, as well as the students' positive perceptions towards the electronic writing portfolio course. Pearson Chi-square analysis also indicated that the distributions of content-, organisation-, and language-related peer writing mediations between the two groups were significantly different which confirmed that the electronic students provided more content-, organisation-, and language-related peer writing mediations during the collaborative writing activities. Practical implications are discussed next.