Borderline and narcissistic personality traits reflect maladaptive patterns of experiencing and behavior, with pronounced difficulties on the interpersonal level. Research shows a relationship between adverse childhood experiences and borderline and narcissistic personality traits. Furthermore, individuals with pronounced traits exhibit a reduced ability to mentalize, i.e., to understand their own and others' mental states, which also develops within the early environment. The aim of this study was to examine whether adverse childhood experiences and the ability to mentalize predict borderline and narcissistic personality traits and the role of mentalization in the relationship between adverse childhood experiences and later borderline and narcissistic traits. The research was conducted online with a sample of 485 participants. The presence of adverse childhood experiences, the ability to mentalize expressed as uncertainty about mental states, and the expression of borderline, grandiose narcissistic, and vulnerable narcissistic personality traits were examined through self-assessment. The results confirmed the hypotheses, indicating that the number of experienced adverse early experiences and uncertainty about mental states are significant predictors of the expression of borderline, grandiose narcissistic, and vulnerable narcissistic personality traits. Additionally, a significant mediating effect of uncertainty about mental states was found in the relationship between adverse childhood experiences and the expression of these traits, with mediation being partial. The research contributes to understanding the development of pathological personality traits, highlights the importance of prevention, and supports mentalization-based therapy.