Computerized cognitive training (CCT) interventions are increasing in their use in outpatient mental health settings. These interventions have demonstrated efficacy for improving functional outcomes when combined with rehabilitation interventions. It has recently been suggested that patients with more cognitive impairment have a greater therapeutic response and that reduced engagement in training can identify cases who manifest low levels of benefit from treatment. Participants were psychiatric rehabilitation clients, with diagnoses of major depression, bipolar disorder and schizophrenia. Newly admitted cases received CCT, delivered via Brain HQ, with cognitive functioning divided into groups on the basis of a BACS t-score of 40 or less vs. more. Training engagement was indexed by the number of training levels achieved per day trained. Forty-nine cases trained on average for 17 days and completed a mean of 150 levels. Overall, patients improved by an average of 4.4 points (0.44 SD) in BACS t-scores ( < .001). Improvements were positively correlated with training engagement ( = 0.30,  < .05), but not with days trained ( = 0.09) or levels earned ( = 0.03) alone. Patients with higher levels of baseline cognitive performance had reduced cognitive gains ( < .003), but did not have less training engagement ( = .97). Diagnoses did not predict cognitive gains ( = .93) or target engagement ( = .74). Poorer performance at baseline and higher levels of training engagement accounted for >10% in independent variance in cognitive gains. The mean level of cognitive improvement far exceeded practice effects. The index of engagement, levels achieved per training day, is easily extracted from the training records of patients, which would allow for early and continuous monitoring of treatment engagement in CCT activities and therapist intervention as needed to improve engagement.