Hospital-wide strategies for reducing mental health (MH) boarding in the emergency departments (ED) have focused on improving the onsite patient flow and resource management. A gap in literature is still residing on the lack of consideration for the MH transition process from the ED into external community and hospital inpatient psychiatric settings, as well as the external bed capacity limitations. Discrete event simulation was used to understand the mental health patient flow within the ED and to inpatient settings. It provided a forecast of system changes when three scenarios were tested to determine the percentage increase in beds necessary to reduce MH ED boarding time. These alternative approaches illustrate the applicability of the model as a decision support tool for evaluating solutions to MH ED boarding.