This study aims to examine the effectiveness of classes in which the teaching-learning design based on Action Learning was employed for adult learners; to this end, this study makes use of sequential mixed method. This study was conducted on 22 people, selected by convenience sampling method, who participated in the Lifelong Education Methodology Course class as a master or doctor degree learner at A graduate school in Gyeonggi Province. First, for quantitative research, a questionnaire was used to conduct pre- and post-tests before and after the class. For qualitative research, reflective journals and questionnaires asking about action learning experienced were used. Further, 4 learners, who were intentionally selected, were in-depth interviewed and analyzed to facilitate the understanding of the results of quantitative research. According to the quantitative research analysis, action learning-based team activities showed statistically significant differences in the efficacy of adult learner s team, understanding of interpersonal relationships and positive problem-solving attitude. In the qualitative research analysis, the team activity participation of adult learners, who were in-depth interviewed, showed various aspects in enhancing learning competency and exploring perceived changes. Specifically, this study could draw and identify 5 topics categorized: enhancing learning effectiveness with positive team activities, improving thinking ability through the acquisition of new learning ways, providing opportunities of listening and communication during the team activity, gaining confidence and a desire for academic growth through enhanced learning motivation, and exploring various learning fields to which continuous lifelong learning will be applied. Therefore, educational strategies should be developed to enable change adult leaners’ perception on the effectiveness of action learning, provide connection and expertise for each field and work experience of current job, promote co-learning and motivation to address on-site problems to enhance capabilities.