Background We aimed to examine the neuropsychological profiles on the Weschler Adult Intelligence Scale (WAIS-IV) of individuals attending a drug and alcohol rehabilitation program and determine the relationship between the multiple domains of cognitive functioning, comorbid mental disorders and treatment completion.Methods One hundred and ninety-six (N = 196) individuals from the Odyssey House Residential Rehabilitation Program, NSW, Australia between 2010-2016 were administered a structured interview including mental and substance use disorders and the Verbal Comprehension (VCI), Perceptual Reasoning (PRI), Working Memory (WMI), and Processing Speed (PSI) domains of the WAIS-IV. Results Verbal Comprehension, Working Memory, and Processing Speed was below the mean relative to normative data derived from the standardisation of the WAIS-IV. Individuals with comorbid mental disorders also displayed significantly lower Verbal Comprehension relative to those without comorbid mental disorders (F = 4.56, p < 0.05). Comorbid psychotic disorders were particularly associated with poor Verbal Comprehension (p < 0.01). Treatment completion was significantly predicted by Verbal Comprehension (p < 0.05).Conclusions Verbal comprehension may be underlying poor treatment completion in drug and alcohol treatment rehabilitation settings. We also highlight an additional relationship between presence of comorbid psychotic disorders and poor verbal comprehension that may be of relevance in planning intervention programs during rehabilitation.