Planning and execution of mass-gathering events involves various challenges. In this case report, the Chicago Model (CM), which was designed to organize and operate such events and to maintain the health and wellbeing of both runners and the public in a more effective way, is described. The Chicago Model also was designed to prepare for unexpected incidents, including disasters, during the marathon event. The model has been used successfully in the planning and execution stages of the Bank of America Shamrock Shuffle and the Bank of America Chicago Marathon since 2008. The key components of the CM are organizational structure, information systems, and communication. This case report describes how the organizers at the 2013 Shamrock Shuffle used the key components of the CM approach in order to respond to an acute incident caused by a man who was threatening to jump off the State Street Bridge. The course route was changed to accommodate this unexpected event, while maintaining access to key health care facilities. The lessons learned from the incident are presented and further improvements to the existing model are proposed.