Background: It is known that psychosocial distress affects the morbidity and mortality of patients with cardiovascular disease of every age. The aim of this study was to produce novel information on how psychological distress can influence cardiovascular performance in patients after cardiac surgery undergoing multidisciplinary cardiac rehabilitation. Methods: Patients (n = 57) admitted after cardiac surgery for valvular or coronary disease underwent, within 5 days of admission, the Symptom Checklist-90-Revised (SCL-90-R) self-report questionnaire to measure psychiatric symptoms and the 12-item General Health Questionnaire (GHQ-12) to assess the level of psychological distress. The Positive Symptom Distress Index (PSDI) was measured to indicate the amplitude of symptom distress. Cardiovascular performance was assessed by a 6 min walking test (6MWT) at admission and discharge, and oxygen consumption (VO2 max) was derived. Results: Within the SCL-90-R score, somatic symptoms (47.4%), depressive and anxiety symptoms (36.8% and 33.3%, respectively), symptoms of phobic anxiety (21.1%), and psychoticism (24.6%) were over-represented. As for the GHQ-12, 75.4% of the sample reported an abnormally negative perception of their health status. An inverse correlation was shown between the variation in 6MWT and SCL depression (p = 0.048), PSDI (p = 0.022), and the GHQ-12 (p = 0.040). Similarly, an inverse correlation was shown between the variation in the VO2 max, GHQ-12 (p = 0.041), and the PSDI (p = 0.023). Conclusions: Post-cardiac surgery cardiac rehabilitation was associated with increased symptoms of psychological discomfort, as compared with the general population. The amplitude of psychological distress, depression, and hostility are associated with limited improvement in performance. These data strengthen the need for psychological support during cardiac rehabilitation programs.