Mentalization ability and cognitive function were compared between individuals with anorexia nervosa (AN) and those with bulimia nervosa (BN). The relationship between body mass index (BMI) and these mental abilities was also explored. Individuals were recruited through an NHS Eating Disorder Service and a national charity offering support and information to people with eating disorders. In total, 110 female participants, 52 with AN and 58 with BN, completed self report measures of eating disorder pathology, mentalization and cognitive ability, along with questionnaires assessing potential confounding variables including demographics, borderline personality disorder, psychological distress, impression management and length of personal therapy received. BMI did not significantly relate to mentalization. Individuals with BN reported having a significantly lower ability to mentalize compared to those with AN, suggesting that some aspect of eating disorder psychopathology, which is primarily associated with BN, may be associated with poor mentalization, regardless of the presence of borderline personality disorder. Whilst no significant difference in overall cognitive impairment was found between the eating disorder groups, low BMI significantly correlated to aspects of cognitive impairment in both groups. This suggests that low BMI should not be ignored as a key risk factor in some aspects of mental processing. Recommendations are made to explore how other aspects of eating disorder psychopathology, such as the feature of impulsivity, the severity of the disorder and other physical changes, may affect a person's ability to understand their internal mental state.