This paper investigates the environmental benefits of integrating a geothermal energy system into St. Patrick's Cathedral in New York City, emphasizing the up-front embodied energy, operational energy savings, and life cycle carbon balance of the system. By replacing traditional HVAC systems with geothermal wells, the cathedral achieved a substantial reduction in energy use intensity, translating into a significant decrease in carbon footprint. The analysis includes a detailed examination of the embodied carbon associated with drilling, materials, and system components, alongside a method for calculating the carbon return on investment (ROI), revealing that the geothermal system's up-front embodied carbon is offset by operational savings within two years. This case study demonstrates the potential practicality and efficiency of geothermal systems in historic buildings while attempting to provide a methodological framework for assessing embodied carbon and the lifecycle carbon balance of geothermal interventions.