Introduction: Retaining clients in residential alcohol and other drug (AOD) treatment is difficult and cognitive impairment has been identified as a significant predictor of treatment dropout. The application of extensive screening for cognitive impairment is cost-prohibitive for most AOD treatment services. The current study aimed to explore cognitive functioning and impairment-associated factors in a typical sample of residential AOD clients using a free brief screening tool that could be utilised by front-line AOD services. Methods: Residents of an AOD therapeutic community (n = 128) and a non-substance using control group (n = 37) were administered a brief cognitive screening measure, the Montreal Cognitive Assessment (MoCA). MoCA total and domain scores were compared between these groups and within the AOD group examined in association with primary substance of misuse, severity of dependence, gender, psychological distress, and history of head injury. Results: Almost half (43.8%) of the AOD sample were identified as cognitively impaired, compared to 16.2% of the control group. Furthermore, 67.2% of the AOD sample had sustained head injuries and 50% of the sample required hospitalization for head injury. History of head injury was a significant determinant of cognitive impairment, and associated with greater levels of psychological distress. Conclusions: There are high rates of inter-related cognitive impairment, head injuries, and psychological distress among clients in residential AOD treatment. Routine screening of clients at intake for cognitive impairment by means of a brief screening measure such as the MoCA, in combination with the assessment of history of head injuries and comorbid psychological disorders, could inform treatment modifications or adjunct interventions to increase retention and improve long-term outcomes.