Introduction and objective: The Kernberg’s concept on the levels of personality organisation enables us to understand, on the one hand, the pathomechanism of personality disorders necessary for case conceptualisation in psychotherapy and, on the other hand, to predict individuals’ everyday functioning in interpersonal relationships and the experienced life satisfaction. In this study, we inquire whether dimensions of pathological personality structure are associated with negative evaluations of life satisfaction and interpersonal relationships. Additionally, we explored whether these relationships are modified by impaired mentalisation (as a mediator) and clinical group membership (as a moderator). Materials and methods: The study involved 88 participants (71 women) from both clinical (n = 45) and non-clinical groups, aged between 20 to 69 years (mean, M = 31.39; standard deviation, SD = 11.38). The participants completed the Kernberg’s Inventory of Personality Organization (IPO), the Relationship Assessment Scale (RAS), the Satisfaction With Life Scale (SWLS), and the Mentalization Questionnaire (MZQ). Results: The results indicate that higher intensity of all dimensions of personality pathology (identity, defence mechanisms, aggression, reality testing, and moral functioning) is significantly associated with lower life satisfaction (rho = from −0.49 to −0.32) and greater difficulties in mentalisation (rho = 0.72 to 0.56). Increased intensity of defence mechanisms is also related to lower romantic relationship satisfaction (rho = −0.23). Moderated mediation analyses with 5000 bootstrap samples (where life satisfaction is an dependent variable, dimensions of personality organisation are independent variables, mentalisation disturbance is a mediator, and clinical group membership moderates all relationships) showed that disrupted mentalisation plays a mediating role in explaining life satisfaction and that clinical vs. non-clinical group membership may not moderate these relationships. Conclusion: The results help specify the relationships between the characteristics of Kernberg’s personality structure dimensions and the evaluation of everyday life satisfaction, suggesting an important role of mentalisation processes as transdiagnostic factors worth addressing in life satisfaction appraisal.