English essay writing is a critical ability for undergraduate students in non-native contexts like Laos, yet many struggle with accuracy and structure. Research purposes were 1) to analyze the nine internal and external factors affecting learners' English essay writing performance, which include writing practice, motivation, coherence and cohesion, writing habit, writing anxiety, vocabulary range, grammar accuracy, writing tools, and instructor feedback and 2) to suggest effective solutions and instructional strategies for overcoming the factors that impede English essay writing among Year III students. Methods: Using a quantitative design, 110 student essays and structured questionnaires were analyzed via multiple regression. Vocabulary range (β = .421, p < .001) and grammar accuracy (β = .287, p = .001) were the strongest predictors of success. While writing anxiety (β = -.149, p = .022) significantly hindered performance, writing habits (β = .195, p = .004) and instructor feedback (β = .173, p = .012) showed positive effects. Motivation and writing resources were not statistically significant. Proficiency and affective factors are vital for Lao EFL learners. The study suggests disciplined practice, positive feedback, and instructional focus on vocabulary and grammar to enhance higher education curricula and student support.