Mentalizing describes the human ability to comprehend one’s own and others’ mental states and is seen as one of the core competencies of psychotherapists. Current research has emphasized the importance of both early dyadic attachment as well as broader sociocultural environmental input on the development of mentalizing. This study investigates whether mentalizing skills, operationalized via reflective functioning (RF), might be influenced by training and working conditions. This study was a matched case-control comparison, cross-sectional study. RF was assessed in a total of 10 psychotherapy trainees working in private practice at the beginning (group A; n = 5) and end (group B; n = 5) of their psychotherapy training (training association: Gestalt Therapy, Institute of Integrative Gestalttherapy Vienna) and in a total of 40 health professionals (institution: General Hospital Vienna—Social Medical Center South, Vienna, Department of Psychiatry, acute psychiatric ward) at the beginning of (group C; n = 20) and without (group D; n = 20) mentalization based therapy training. The participants differed from each other regarding their training, but participants of the same institution were matched. RF scores were significantly higher in group A and B than in group C and D (A,C: p = 0.0065, Odds Ratio (OR): 0.0294; A,D: p = 0.0019, OR: 0.0132; B,C: p = 0.0065, OR: 0.0294, B,D: p = 0.0019, OR: 0.0132). RF scores were not significantly different among groups A and group B (A,B: p > 0.9999) or between groups C and D (C,D: p = 0.6050). The current study suggests that mentalizing skills might be rather slow to improve by training, but that they might be influenced by the context.