<p>Postdocs encounter numerous challenges as they transition from students to independent scholars, with the sustainability of their academic careers often marked by uncertainty and obstacles. While the career challenges faced by postdocs have attracted considerable attention, research on their career development from a sustainable perspective remains limited. Given China’s increasing emphasis on building scientific and technological talent to drive the development of New Quality Productive Forces, this study aims to explore the relationship between mentor support and sustainable careers, focusing on the roles of work engagement and academic career identity among Chinese postdocs based on Conservation of Resources theory. Based on analyses of 343 questionnaires from Chinese postdocs, the results indicate that mentor support is significantly and positively associated with postdocs’ employability, subjective career success, and mental health—three key indicators of sustainable careers. The mediation effect analyses demonstrate that work engagement and academic career identity simultaneously and sequentially mediate the relationship between mentor support and sustainable careers. This study not only offers new insights into how mentor support, as an essential job resource, contributes to postdoctoral career sustainability, complementing the existing literature on mentor support and sustainable careers, but also provides valuable practical guidance for postdoctoral stations on how to enhance postdoctoral career sustainability by strengthening the role of mentors.</p>