Background: Mental illness correlates with an increased length of stay (LOS) for patients hospitalized for medical conditions. While psychiatric consultations help manage mental illness among those hospitalized for medical conditions, consultations initiated by nonpsychiatric mental disease may lack maximum effectiveness. Methods: In a before-and-after design, in 2 contiguous years LOS for internist-initiated, conventional consultation (CC) as usual treatment was compared to LOS of a proactive, mental health professional-initiated, multidisciplinary intervention delivered by the behavioral intervention team (BIT) on the same units. The patient populations included general medical patients with a variety of illnesses. Patients were treated in 3 different inpatient settings with a total capacity of 92 beds serving 15,858 patient visits over 3 comparison years. BIT comprised a psychiatrist, a nurse, and a social worker, each of whom performed the specific tasks of their professional discipline, while collaborating among themselves and their health-care colleagues. BIT provided timely, appropriate, and effective patient care alongside consultative advice and education to their corresponding professional peers. BIT was compared to CC on the outcome of LOS. Results: There was a statistically significant reduction of LOS favoring BIT over CC for patients with an LOS of <31 days which persisted while controlling for multiple co-morbid factors. Also, a statistically significant spillover effect was suggested by the overall improvement of LOS on units implementing BIT. Conclusion: BIT is a promising means of lowering LOS on general medical units while providing a high level of care and staff support.