Portfolio assessment is considered to be one of the more advanced approaches to enhancing the proficiency of English among second language (ESL) students. The research examines how ESL learners' writing processes involved in portfolio creation influence their overall writing performance, specifically focusing on electronic portfolios (EPs) and paper-based portfolios (PPs). The research also identifies the challenges that Saudi ESL learners face when using the writing portfolio. A mixed-method approach was adopted for the research. The quantitative method consists of ESL tests distributed among one hundred and twenty male ESL learners at Yanbu Industrial College in Saudi Arabia to determine the effect of various writing portfolios on students’ writing performance. A qualitative method, which consists of classroom observations, document analysis, and interviews with twelve ESL students and seven ESL teachers from different universities in Saudi Arabia, has also been utilized. The research highlights the fact that although there is no significant difference between paper-based (PPs) portfolios and electronic portfolios (EPs) in developing students' performance in writing, specific pedagogical approaches should be implemented in assessing the writing portfolios, such as the students’ reflections and their peers' assessments, to raise their awareness of language structures. ESL teachers should also adopt a correction method that only highlights common mistakes to help students notice their errors and avoid charging them with negative feelings of excessive corrections. The research’s outcome could raise ESL teachers’ awareness of the best pedagogical methods of evaluating the writing portfolio and provide some strategies to help students develop their writing performance.