Exercise GTA Unified was a functional, multi-agency, cross-jurisdictional, health-sector focused mass casualty preparedness exercise conducted in the Greater Toronto Area (GTA) on 28th November, 2019. With over 1,000 unique paper-based and electronic injects and 34 participating agencies, including 22 separate hospital sites, Exercise GTA Unified is likely the largest health-sector focused mass casualty preparedness exercise ever conducted in Canada. The exercise design approach supported a successful, objective-based functional exercise, with elements of marked realism for participants. The exercise offered a unique opportunity to collect data for future analysis and the insights gained will have a transformative impact on interagency engagement and cooperation for emergency response planning. Furthermore, the approach adopted for the exercise is affordable, reproducible, scalable and transferrable to sectors beyond the health system. This paper provides a detailed review of the key planning and design components adopted in the development and implementation of the exercise, as well as practical insights for the design and conduct of multi-agency, cross-jurisdictional functional exercises.