The majority of the numerous train-the-coach courses in the growing coaching market are not evidence-based and evaluate neither the personal development of coaches during these courses nor their effects on clients’ subsequent coaching success. The aim of our study was to investigate the development of coaches’ career-related variables during a train-the-coach course. Furthermore, we explored the effects of these coach variables on clients’ coaching success. Fifty-seven German university students participated in a five-month train-the-coach course and answered questionnaires regarding career-related variables at three points of time during the course. Afterwards, in groups of two or three, they led group coachings for the vocational orientation of secondary school pupils. The pupils (N = 104) answered questionnaires at the beginning, in the middle and at the end of the 10-week coaching process. Results indicated that coaches’ occupational self-efficacy, goal orientation and career adaptability significantly increased during the train-the-coach course. Furthermore, higher levels of coaches’ goal orientation and career adaptability led to stronger increases of clients’ career decisionmaking self-efficacy during the coaching process. This might be due to coaches’ role modelling behaviour. Our findings suggest that coaches not only gain technical competences during trainthe-coach courses, but also improve in important career-related variables. Thus, train-the-coach courses might contribute to the personal development of university students. Additionally, coaches’ career-related variables indeed seem significant for clients’ coaching success and should therefore be encouraged during coach qualification.