Background: Adults with a history of childhood maltreatment are more likely to experience distrust, feel distant from others, and develop an insecure attachment style which may also affect relationship quality. Furthermore, childhood maltreatment has been linked to several mental health problems; including, depression, anxiety, and alcohol dependance severity, that are also known to relationship quality. Objective: The current study was designed to investigate to what extent childhood maltreatment is associated with adult insecure attachment and intimate relationships and whether this association is mediated by psychopathology. Participants and Method: In a study comprised of 2035 adults aged 18–65, we investigated whether childhood maltreatment was associated with insecure adult attachment styles and the quality of intimate relationships and whether this was mediated by depression, anxiety, and alcohol dependence severity (based on repeated assessments of the Inventory of Depressive Symptomatology-Self Report, Beck Anxiety Index, and Alcohol Use Disorders Identification Test respectively). Results: The path model showed an acceptable fit, RMSEA = 0.05, and suggested full mediation of the association of childhood maltreatment with quality of intimate relationships by depression severity and a) anxious attachment (β = −4.0 ∗ 10−2; 95% CI = −5.5 ∗ 10−2, −2.7 ∗ 10−2) and b) avoidant attachment (β = −7.2 ∗ 10−2; 95% CI = −9.6 ∗ 10−2, −4.9 ∗ 10−2). Anxiety and alcohol dependence severity were not significant mediators. Conclusions: Childhood maltreatment is associated with a lower quality of intimate relationships, which is fully mediated by depression severity and insecure attachment styles.