We conducted this realist evaluation study of an organizational health intervention involving 421 low-skilled workers (50% female), half of whom were immigrants, in three companies over six months. Non-profit agencies implemented peer-mentoring and taught peer-mentors and line managers how to enhance social support in order to improve workers’ work situation in a participative way. We formulated five mechanisms of change: the company management encouragement mechanism, the role model mechanism, the peer-mentor support mechanism, the line manager support mechanism, and the participative work improvement mechanism. We combined realist evaluation with a quasi-experimental design and process evaluation in a multi-methods approach. Results of multiple group latent change models and qualitative research showed that intervention-group workers perceived increases in peer-mentor support but not in line manager support. Peer-mentors managed to initiate high-quality improvements at work. Intervention-group workers showed significant reductions in blood pressure. Control-group workers experienced more psychosomatic complaints over time in significant contrast to intervention-group workers. Our results suggest that peer-mentoring offers an effective way for low-skilled workers and immigrants to achieve better health. To improve such health effects, a greater focus on line managers’ work situations is needed to help them provide support.