Urban ghost spaces have potential to reduce flooding in the Lower East Don Watershed by reducing and delaying stormwater runoff to receiving waterways. Ghost spaces are vacant or derelict space that have prior commercial, industrial, transportation, or residential use, receive semi-regular maintenance, and are unused. Past studies emphasize the importance of metropolitan place-based studies for flood mitigation research and highlight the opportunities vacant space presents. Geographic Information System (GIS) was used to locate 1,863 ghost spaces for Low Impact Development (LID), and a Stormwater Management Model (PC-SWMM) to quantify runoff captured within selected ghost spaces. Within identified ghost spaces stormwater runoff volume was reduced by 85% for 2-year, 5-year, and 10-year rainfall events. A strategic selection of 73 opportune ghost spaces, which focus on retention-based LIDs, reduced runoff by 93%. Defining an interconnected system of ghost spaces demonstrates an opportunity to advance flood mitigation strategies across Toronto, Ontario.