The primary factor that significantly contributes to the increased prevalence of academic underachievement among Pakistani students in public universities is the inadequate level of writing proficiency. Students encounter various challenges in English writing, such as a limited vocabulary repertoire, incorrect grammatical usage, insufficient motivation, an unsupportive educational setting, and a lack of self-assurance. The primary aim of this study is to investigate the impact of metacognitive writing strategy, specifically the act of evaluating one's own writing, on writing performance among Pakistani ESL learners. Additionally, this study seeks to explore the potential mediating role of willingness to write in this relationship. The purpose of this research is to address the issue of subpar writing performance and contribute to the existing literature on writing strategies in second language acquisition. The research design utilised in this study was a mixed-methods approach, incorporating both a survey questionnaire and semi-structured interviews. These methods were employed to gather data from students enrolled in BS English programmes at public universities in South Punjab, Pakistan. The quantitative data analysis for the study was conducted using Smart PLS 3, while the qualitative data analysis employed thematic analysis. The findings of the research underscored the beneficial impact of utilising evaluation as a metacognitive writing strategy and the level of motivation to write on the writing proficiency of Pakistani English as a Second Language (ESL) learners. The present investigation holds significant practical implications for students, educators, policymakers, and curriculum developers in their efforts to enhance the writing proficiency of English as a Second Language (ESL) learners by implementing suitable strategies.