This mixed-methods research study aims to investigate the factors influencing EFL teachers to leave or consider leaving their teaching profession. Forty-eight participants at the language institute of one public university in Thailand including 29 currently in-service Thai teachers, 13 currently in-service foreign teachers, 3 retired Thai teachers, 2 Thai teachers leaving teaching careers, and 1 Thai teachers moving to another institution completed the questionnaire. Twenty-three of them including 10 currently in-service Thai teachers, 7 currently in-service foreign teachers, 3 retired Thai teachers, 2 Thai teachers leaving teaching careers, and 1 Thai teachers moving to another institution gave an interview. Employment, external, and personal factors are found to be factors influencing teacher attrition. Demotivating work policies, inappropriate salary and benefits, negative characteristics of the profession and family responsibilities are believed to be reasons why EFL teachers leave or consider leaving their teaching career. This study has implications in lowering teacher attrition since understanding the factors of teacher attrition helps policy makers and practitioners prioritize and design teacher retention strategies.