Human agency theory has been known for more than two decades, but the applications in EFL learning are still insufficiently researched and the roles in enhancing English proficiency are still little explored. This study, thus, examined learners' agency in English learning and investigated the role in English proficiency. Using a quantitative research design, it collected data from 13 non-English disciplines of schools at a university in the south of Thailand. The total number of participants was 389 undergraduate students (22.4% male; 76.9% female; .8% prefer not to say) involving 43.4% 2nd and 56.6% 1st-year students with levels of proficiency ranging from A1 to B1 in the Common European Framework of Reference for Languages (CEFR). The data were analysed by using descriptive statistics, independent t-test, one-way ANOVA, bivariate correlation, multiple linear regression, and mediation analysis. The results indicated a higher level of learners' agency for learning with some variations by year of study and proficiency level. Thai EFL learners' agency for learning had predictive roles on their proficiency, yet no potential mediators were observed.