This research aims to understand language learning strategies of Thai adult learners and factors affecting their strategy use. The participants are forty officers of General Service Division of the Council of State of Thailand, attending an English training course for developing their work potential. The data were collected through the questionnaire adapted from the Strategy Inventory for Language Learning (SILL). To further explore personal views about their experience in learning strategies and factors influential on their strategy use, fifteen participants were selected for individual interviews. Findings revealed that the learners reported an overall preference for the use of social strategies. Analysis of the qualitative data confirmed most of the SILL responses and revealed additional strategies and factors affecting the strategy use. Individual learners chose strategies suitable for the achievement of their goals and take account of their affective needs and work context. Overall, this study has emphasized the necessity of having qualitative data which can enrich and illuminate the findings of quantitative data and could be valuable resources for considering appropriate ways in which English proficiency of Thai adult learners could be developed. Implications are drawn regarding the language learning strategies of adult learners and their strategy use as professional engagement as well as recommendations for future research.