Background The beginning of the career of teachers is a demanding phase. In the combined roles of teacher and trainee, and in the shift from academic to practical learning, trainee teachers have to acquire competencies in teaching as well as coping with stress. In this phase, the phenomenon of “reality shock” is widely observed. Objective A mindfulness training was developed to support teacher trainees during their first year. This intervention study investigated perceived and physiological stress at teachers’ career start and the stress reducing effects of the mindfulness training in this phase. Methods In a quasi-experimental design, 19 out of 42 participants from this sample underwent mindfulness-based stress reduction training and a wait-list control group (N = 23) underwent a compact course after post measurements. We measured physiological stress parameters and perceived stress at 3 different time points. Heart rate signals were acquired in ambulatory assessment sequences, including teaching, rest periods, and cognitive tasks. The data were analyzed in linear mixed-effects models. Results We found high physiological stress in the very beginning of teacher training, which attenuated over time. The mindfulness intervention only led to a greater reduction in heart rate ( d = .53 to .74) in situations where the intervention group had shown higher heart rate levels initially, but not in heart rate variability. However, the mindfulness group significantly reduced ( d = .63) their perceived stress and maintained ( d = .55) this improvement, while the control group maintained a highly elevated perceived stress level throughout. Conclusion The mindfulness training could reduce subjective stress, which otherwise seems to be a long-lasting aspect of beginning teachers’ “reality shock”. Indications of a superior reduction of physiological stress in demanding situations were weak, while generally, undue physiological stress seems to be a temporary phenomenon in the initial phase of teacher induction.