The purpose of this study was to examine the mediating effect of informal learning activity on the relationship between protean career attitude and subjective career success in lifelong educators. For doing so, an online survey was conducted with qualified lifelong educators working at lifelong education institutions in Korea, from Jan. 11, 2019 to Feb. 14, 2019. Total number of 146 responded to this survey, but 140 responses were analyzed for this study except for incomplete responses. The results of this study were as follows: First, lifelong educators' protean career attitude, informal learning activity, and subjective career success had positive correlation. Second, lifelong educators' protean career attitude had a direct effect on subjective career success, while informal learning activity had a mediating effect between protean career attitude and subjective career success. Based on these results, this study showed that lifelong educators kept pursuing professional career attitude with their own values and tried to make career management on their own direction. In addition, this study showed that it must be also necessary for lifelong educators to actively participate in various informal learning activities, as those efforts would reinforce a cause-effect relation between protean attitude and subjective career success. Finally, several implications were discussed in terms of theoretical and practical perspectives.